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+[[!meta copyright="Copyright © 2010, 2012, 2013 Free Software Foundation,
+[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
+id="license" text="Permission is granted to copy, distribute and/or modify this
+document under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; with no Invariant
+Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license
+is included in the section entitled [[GNU Free Documentation
+[[!tag open_issue_gnumach open_issue_glibc]]
+This issue has been known for some time, due to coreutils' testsuite choking
+when testing *nice*: [[!debbug 190581]].
+There has been older discussion about this, too, but this is not yet captured
+# IRC, freenode, #hurd, 2010-08
+ <pochu> I'm reading Mach and POSIX documentation to understand the priorities/nice problems
+ <pochu> antrik said it would be better to reimplement everything instead of
+ fixing the current Mach interfaces, though I'm not sure about that yet
+ <youpi> uh, so he changed his mind?
+ <pochu> it seems POSIX doesn't say nice values should be -20..20, but
+ 0..(2*NZERO - 1)
+ <youpi> he said we could just change the max priority value and be done
+ with it :)
+ <pochu> so we can probably define NZERO to 16 to match the Mach range of
+ 0..31
+ <youpi> s/said/had said previously/
+ <antrik> youpi: POSIX is actually fucked up regarding the definition of
+ nice values
+ <antrik> or at least the version I checked was
+ <pochu> antrik: why? this says the range is [0,{NZERO}*2-1], so we can just
+ set NZERO to 16 AFAICS:
+ <antrik> it talkes about NZERO and all; making it *look* like this could be
+ defined arbitrarily... but in other places, it's clear that the standard
+ 40 level range is always assumed
+ <antrik> anyways, I totally see no point in deviating from other systems in
+ this regard. it can only cause problems, and gives us no benefits
+ <cfhammar> it says NZERO should be at least 20 iirc
+ <youpi> agreed
+ <antrik> I don't remember the details; it's been a while since I looked at
+ this
+ <antrik> youpi: changing the number of levels is only part of the
+ issue. I'm not sure why I didn't mention it initially when we discussed
+ this
+ <antrik> youpi: I already concluded years ago that it's not possible to
+ implement nice levels correctly with the current Mach interfaces in a
+ sane fashion
+ <antrik> (it's probably possible, but only with a stupid hack like setting
+ all the thread priorities one by one)
+ <antrik> youpi: also, last time we discussed this, I checked how the nice
+ stuff works currently on Hurd; and concluded that it's so utterly broken,
+ that there is no point in trying to preserve *any* compatibility. I think
+ we can safely throw away any handling that is alread there, and do it
+ over from scratch in the most straightforward fashion
+ <pochu> antrik: I've thought about setting NZERO to 16 and doing exactly
+ what you've just said to be a hack (setting all the thread priorities one
+ by one)
+ <pochu> but there seems to be consensus that that's undesirable...
+ <pochu> indeed, POSIX says NZERO should be at least 20
+ <antrik> pochu: BTW, I forgot to say: I'm not sure you appreciate the
+ complexity of setting the thread max priorities individually
+ <pochu> antrik: I don't. would it be too complex? I imagined it would be a
+ simple loop :)
+ <antrik> pochu: in order to prevent race conditions, you have to stop all
+ other threads before obtaining the list of threads, and continue them
+ after setting the priority for each
+ <antrik> I don't even know whether it can be done without interfering with
+ other thread handling... in which case it gets really really ugly
+ <pochu> antrik: btw I'm looking at [gnumach]/kern/thread.[ch], removing the
+ priority stuff as appropriate, and will change the tasks code later
+ <antrik> it seems to me that using a more suitable kernel interface will
+ not only be more elegant, but quite possibly actually easier to
+ implement...
+ <pochu> antrik: apparently it's not that hard to change the priority for
+ all threads in a task, see task_priority() in gnumach/kern/task.c
+ <pochu> it looks like the nice test failures are mostly because of the not
+ 1:1 mapping between nice values and Mach priorities
+ <marcusb> "Set priority of task; used only for newly created threads."
+ <marcusb> there is a reason I didn't fix nice 8 years ago
+ <marcusb> ah there is a change_threads option
+ <pochu> marcusb: I'm not sure that comment is correct. that syscall is used
+ by setpriority()
+ <marcusb> yeah
+ <marcusb> I didn't read further, where it explains the change_threads
+ options
+ <marcusb> I was shooting before asking questions :)
+ <marcusb> pochu: although there are some bad interactions if max_priorities
+ are set per thread
+ <antrik> pochu: maybe we are talking past each other. my point was not that
+ it's hard to do in the kernel. I was just saying that it would be painful
+ to do from userspace with the current kernel interface
+ <pochu> antrik: you could still use that interface in user space, couldn't
+ you? or maybe I'm misunderstanding...
+ <pochu> cfhammar, antrik: current patch:
+, main issue is probably what
+ to do with high-priority threads. are there cases where there should be a
+ thread with a high priority but the task's priority shouldn't be high?
+ e.g. what to do with kernel_thread() in [gnumach]/kern/thread.c
+ <pochu> i.e. if tasks have a max_priority, then threads shouldn't have a
+ higher priority, but then either we raise the task's max_priority if we
+ need a high-prio thread, or we treat them specially (e.g. new field in
+ struct thread), or maybe it's a non-issue because in such cases, all the
+ task is high-prio?
+ <pochu> also I wonder whether I can kill the processor set's
+ max_priority. It seems totally unused (I've checked gnumach, hurd and
+ glibc)
+ <pochu> (that would simplify the priority handling)
+ <cfhammar> pochu: btw what does your patch do? i can't remember what was
+ decided
+ <pochu> cfhammar: it moves the max_priority from the thread to the task, so
+ raising/lowering it has effect on all of its threads
+ <pochu> it also increases the number of run queues (and thus that of
+ priority levels) from 32 to 40 so we can have a 1:1 mapping with nice
+ values
+ <pochu> cfhammar: btw don't do a full review yet, just a quick look would
+ be fine for now
+ <neal> why not do priorities from 0 to 159
+ <neal> then both ranges can be scaled
+ <neal> without loss of precision
+ <pochu> neal: there would be from Mach to nice priorities, e.g. a task with
+ a priority of 2 another with 3 would have the same niceness, though their
+ priority isn't really the same
+ <neal> pochu: sure
+ <neal> pochu: but any posix priority would map to a current mach priority
+ and back
+ <neal> sorry, that's not true
+ <neal> a posix priority would map to a new mach priority and bach
+ <neal> and a current mach priority would map to a new mach priority and
+ back
+ <neal> which is I think more desirable than changing to 40 priority levels
+ <pochu> neal> and a current mach priority would map to a new mach priority
+ and back <- why should we care about this?
+ <neal> to be compatible with existing mach code
+ <neal> why gratutiously break existing interfaces?
+ <pochu> they would break anyway, wouldn't them? i.e. if you do
+ task_set_priority(..., 20), you can't know if the caller is assuming old
+ or new priorities (to leave it as 20 or as 100)
+ <neal> you add a new interface
+ <neal> you should avoid changing the semantics of existing interfaces as
+ much as possible
+ <pochu> ok, and deprecate the old ones I guess
+ <neal> following that rule, priorities only break if someone does
+ task_set_priority_new(..., X) and task_get_priority ()
+ <neal> there are other users of Mach
+ <neal> I'd add a configure check for the new interface
+ <neal> alternatively, you can check at run time
+ <pochu> well if you _set_priority_new(), you should _get_priority_new() :)
+ <neal> it's not always possible
+ <pochu> other users of GNU Mach?
+ <neal> you are assuming you have complete control of all the code
+ <neal> this is usually not the case
+ <neal> no, other users of Mach
+ <neal> even apple didn't gratuitously break Mach
+ <neal> in fact, it may make sense to see how apple handles this problem
+ <pochu> hmm, I hadn't thought about that
+ <pochu> the other thing I don't understand is: "I'd add a configure check
+ for the new interface". a configure check where? in Mach's configure?
+ that doesn't make sense to me
+ <neal> any users of the interface
+ <pochu> ok so in clients, e.g. glibc & hurd
+ <neal> yes.
+ <antrik> neal: I'm not sure we are winning anything by keeping
+ compatibility with other users of Mach...
+ <antrik> neal: we *know* that to make Hurd work really well, we have to do
+ major changes sooner or later. we can just as well start now IMHO
+ <antrik> keeping compatibility just seems like extra effort without any
+ benefit for us
+ <guillem> just OOC have all other Mach forks, preserved full compatibility?
+ <neal> guillem: Darwin is pretty compatible, as I understand it
+ <antrik> pochu: the fundamental approach of changing the task_priority
+ interface to serve as a max priority, and to drop the notion of max
+ priorities from threads, looks fine
+ <antrik> pochu: I'm not sure about the thread priority handling
+ <antrik> I don't know how thread priorities are supposed to work in chreads
+ and/or pthread
+ <antrik> I can only *guess* that they assume a two-stage scheduling
+ process, where the kernel first decides what process to run; and only
+ later which thread in a process...
+ <antrik> if that's indeed the case, I don't think it's even possible to
+ implement with the current Mach scheduler
+ <antrik> I guess we could work with relative thread priorities if we really
+ want: always have the highest-priority thread run with the task's max
+ priority, and lower the priorities of the other threads accordingly
+ <antrik> however, before engaging into this, I think you should better
+ check whether any of the code in Hurd or glibc actually uses thread
+ priorities at all. my guess is that it doesn't
+ <antrik> I think we could get away with stubbing out thread priority
+ handling alltogether for now, and just use the task priority for all
+ threads
+ <antrik> I agree BTW that it would be useful to check how Darwin handles
+ this
+ <pochu> btw do you know where to download the OS X kernel source? I found
+ something called xnu, but I?m not sure that's it
+ <antrik> pochu: yeah, that's it
+ <antrik> Darwin is the UNIX core of OS X, and Xnu is the actual kernel...
+ <pochu> hmm, so they have both a task.priority and a task.max_priority
+ <neal> pochu: thoughts?
+ <pochu> neal: they have a priority and a max_priority in the task and in
+ the threads, new threads inherit it from its parent task
+ <pochu> then they have a task_priority(task, priority, max_priority) that
+ can change a task's priorities, and it also changes it for all its
+ threads
+ <neal> how does the global run queue work?
+ <pochu> and they have 128 run queues, no idea if there's a special reason
+ for that number
+ <pochu> neal: sorry, what do you mean?
+ <neal> I don't understand the point of the max_priority parameter
+ <pochu> neal: and I don't understand the point of the (base) priority ;)
+ <pochu> the max_priority is just that, the maximum priority of a thread,
+ which can be lowered, but can't exceed the max one
+ <pochu> the (base) priority, I don't understand what it does, though I
+ haven't looked too hard. maybe it's the one a thread starts at, and must
+ be <= max_priority
+ <antrik> pochu: it's clearly documented in the manual, as well as in the
+ code your initial patch changes...
+ <antrik> or do you mean the meaning is different in Darwin?...
+ <pochu> I was speaking of Darwin, though maybe it's the same as you say
+ <antrik> I would assume it's the same. I don't think there would be any
+ point in having the base vs. max priority distinction at all, except to
+ stay in line with standard Mach...
+ <antrik> at least I can't see a point in the base priority semantics for
+ use in POSIX systems...
+ <pochu> right, it would make sense to always have priority == max_priority
+ ...
+ <pochu> neal: so max_priority is that maximum priority, and priority is the
+ one used to calculate the scheduled priority, and can be raised and
+ lowered by the user without giving special permissions as long as he
+ doesn't raise it above max_priority
+ <pochu> well this would allow a user to lower a process' priority, and
+ raise it again later, though that may not be allowed by POSIX, so then we
+ would want to have max_priority == priority (or get rid of one of them if
+ possible and backwards compatible)
+ <antrik> pochu: right, that's what I think too
+ <antrik> BTW, did I bring up handling of thread priorities? I know that I
+ meant to, but I don't remember whether I actually did...
+ <pochu> antrik: you told me it'd be ok to just get rid of them for now
+ <pochu> so I'm more thinking of fixing max_priority and (base) priority and
+ leaving thread's scheduling priority as it currently is
+ <pochu> s/so/though/
+ <antrik> pochu: well, my fear is that keeping the thread priority handling
+ as ist while changing task priority handling would complicate the
+ changes, while giving us no real benefit...
+ <antrik> though looking at what Darwin did there should give you an idea
+ what it involves exactly...
+ <pochu> antrik: what would you propose, keeping sched_priority ==
+ max_priority ?
+ <pochu> s/keeping/making/
+ <antrik> yes, if that means what I think it does ;-)
+ <antrik> and keeping the priority of all threads equal to the task priority
+ for now
+ <antrik> of course this only makes sense if changing it like this is
+ actually simpler than extending the current handling...
+ <antrik> again, I can't judge this without actually knowing the code in
+ question. looking at Darwin should give you an idea...
+ <pochu> I think leaving it as is, making it work with the task's
+ max_priority changes would be easier
+ <antrik> perhaps I'm totally overestimating the amount of changes required
+ to do what Darwin does
+ <antrik> OTOH, carrying around dead code isn't exactly helping the
+ maintainability and efficiency of gnumach...
+ <antrik> so I'm a bit ambivalent on this
+ <antrik> should we go for minimal changes here, or use this occasion to
+ simplify things?...
+ <antrik> I guess it would be good to bring this up on the ML
+ <cfhammar> in the context of gsoc i'd say minimal changes
+ <pochu> there's also neal's point on keeping backwards compatibility as
+ much as possible
+ <neal> my point was not backwards compatibility at all costs
+ <antrik> I'm still not convinced this is a valid point :-)
+ <neal> but to not gratutiously break things
+ <antrik> neal: well, I never suggested breaking things just because we
+ can... I only suggested breaking things to make the code and interface
+ simpler :-)
+ <antrik> I do not insist on it though
+ <neal> at that time, we did not know how Mac did it
+ <antrik> I only think it would be good to get into a habit that Mach
+ interfaces are not sacred...
+ <neal> and, I also had a proposal, which I think is not difficult to
+ implement given the existing patch
+ <antrik> but as I said, I do not feel strongly about this. if people feel
+ more confident about a minimal change, I'm fine with that :-)
+ <antrik> neal: err... IIRC your proposal was only about the number of nice
+ levels? we are discussing the interface change necessary to implement
+ POSIX semantics properly
+ <antrik> or am I misremembering?
+ <pochu> antrik: he argues that with that number of nice levels, we could
+ keep backwards compatibility for the 0..31 levels, and for 0..39 for
+ POSIX compatibility
+ <antrik> pochu: yes, I remember that part
+ <neal> antrik : My suggestion was: raise the number of nice levels to 160
+ and introduce a new interface which uses those. Adjust the old interface
+ to space by 160/32
+ <antrik> neal: I think I said it before: the problem is not *only* in the
+ number of priority levels. the semantics are also wrong. which is why
+ Darwin added a max_priority for tasks
+ <neal> what do you mean the semantics are wrong?
+ <neal> I apologize if you already explained this.
+ <antrik> hm... I explained it at some point, but I guess you were not
+ present at that conversation
+ <neal> I got disconnected recently so I likely don't have it in backlog.
+ <antrik> in POSIX, any process can lower its priority; while only
+ privileged processes can raise it
+ <antrik> Mach distinguishes between "current" and "max" priority for
+ threads: "max" behaves like POSIX; while "current" can be raised or
+ lowered at will, as long as it stays below "max"
+ <antrik> for tasks, there is only a "current" priority
+ <antrik> (which applies to newly created threads, and optionally can be set
+ for all current threads while changing the task priority)
+ <antrik> glibc currently uses the existing task priorities, which leads to
+ *completely* broken semantics
+ <antrik> instead, we need something like a max task priority -- which is
+ exactly what Darwin added
+ <neal> yes
+ <antrik> (the "current" task priority is useless for POSIX semantics as far
+ as I can tell; and regarding thread priorities, I doubt we actually use
+ them at all?...)
+ <cfhammar> where does a new thread get its initial max_priority from?
+ <antrik> cfhammar: from the creator thread IIRC
+ <pochu> yes
+## IRC, freenode, #hurd, 2010-08-12
+ <pochu> my plan is to change the number of priority levels and the
+ threads/tasks priority handling, then add new RPCs to play with them and
+ make the old ones stay compatible, then make glibc use the new RPCs
+# IRC, freenode, #hurd, 2012-12-29
+ <braunr> and, for a reason that i can't understand, there are less
+ priorities than nice levels, despite the fact mach was designed to run
+ unix systems on top of it ..
+ <youpi> btw, didn't we have a plan to increase that number?
+ <braunr> i have no idea
+ <braunr> but we should :)
+ <youpi> I remember some discussion about it on the list
+## IRC, freenode, #hurd, 2012-12-31
+ <youpi> braunr: btw, we *do* have fixed the nice granularity
+ <youpi> +#define MACH_PRIORITY_TO_NICE(prio) ((prio) - 20)
+ <youpi> in the debian package at least
+ <youpi> ah, no
+ <youpi> it's not applied yet
+ <youpi> so I have the patch under hand, just not applied :)
+ <braunr> but that's a simple shift
+ <braunr> the real problem is that there aren't as many mach priorities as
+ there are nice levels
+ <youpi> that's not really a problem
+ <youpi> we can raise that in the kernel
+ <youpi> the problem is the change from shifted to unshifted
+ <youpi> that brings odd nice values during the upgrade
+ <braunr> ok
+ <braunr> i hope the scheduler code isn't allergic to more priorities :)
+## IRC, freenode, #hurd, 2013-01-02
+ <braunr> pochu: i see you were working on nice levels and scheduling code
+ some time ago
+ <braunr> pochu: anything new since then ?
+ <pochu> braunr: nope
+ <braunr> pochu: were you preparing it for the gsoc ?
+ <pochu> braunr: can't remember right now, either that or to fix a ftbfs in
+ debian
+ <youpi> iirc it's coreutils which wants proper nice levels
+# IRC, OFTC, #debian-hurd, 2013-03-04
+ <Steap> Is it not possible to set the priority of a process to 1 ?
+ <Steap> these macros:
+ <Steap> #define MACH_PRIORITY_TO_NICE(prio) (2 * ((prio) - 12))
+ <Steap> #define NICE_TO_MACH_PRIORITY(nice) (12 + ((nice) / 2))
+ <Steap> are used in the setpriority() implementation of Hurd
+ <Steap> so setting a process' priority to 1 is just like setting it to 0
+ <youpi> Steap: that has already been discussed to drop the *2
+ <youpi> the issue is mach not supporting enough sched levels
+ <youpi> can be fixed, of course
+ <youpi> just nobody did yet
+GNU Mach commit 6a234201081156e6d5742e7eeabb68418b518fad (and commit
+## IRC, OFTC, #debian-hurd, 2013-03-07
+ <braunr> youpi: btw, why did you increase the number of priorites to 50 ?
+ <youpi> for the nice levels
+ <braunr> and probably something more, there are only 40 nice levels
+ <youpi> yes, the current computation leaves some margin
+ <youpi> so I preferred to keep a margin too
+ <braunr> ok
+ <youpi> e.g. for the idle thread, etc.
+ <braunr> or interrupt threads
+ <youpi> yep
+ <braunr> i see the point, thanks
+ <tschwinge> Is the number of 40 specified by POSIX (or whatever) or is that
+ a Linuxism?
+ <braunr> good question
+ <braunr> posix is weird when it comes to such old unixisms
+ <braunr> there is a NZERO value, but i don't remember how it's specified
+ <youpi> it's at least 20
+ <tschwinge> (I don't object to the change; just wondered. And if practice,
+ you probably wouldn't really need more than a handful. But if that
+ change (plus some follow-up in glibc (?) improves something while not
+ adding a lot of overhead, then that's entirely fine, of course.)
+ <braunr> "A maximum nice value of 2*{NZERO}-1 and a minimum nice value of 0
+ shall be imposed by the system"
+ <braunr> NZERO being 20 by default
+ <youpi> and 20 is the minimum for NZERO too
+ <braunr> hm, not the default, the minimul
+ <braunr> minimum
+ <braunr> yes that's it
+ <braunr> ok so it's actually well specified
+ <tschwinge> Aha, I see (just read it, too). So before that change we
+ simply couldn't satisfy the POSIX requirement of (minimum) NZERO = 20,
+ and allowing for step-1 increments. Alright.
+ <youpi> yep
+ <youpi> thus failing in coreutils testsuite