From: Oleg Nesterov <oleg@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sasha Levin <sasha.levin@oracle.com>,
Frederic Weisbecker <fweisbec@gmail.com>,
Ingo Molnar <mingo@kernel.org>, Peter Anvin <hpa@zytor.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Andy Lutomirski <luto@amacapital.net>,
Denys Vlasenko <dvlasenk@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Chuck Ebbert <cebbert.lkml@gmail.com>
Subject: Re: [tip:x86/asm] x86: Speed up ___preempt_schedule*() by using THUNK helpers
Date: Sat, 4 Oct 2014 02:19:23 +0200 [thread overview]
Message-ID: <20141004001923.GA5539@redhat.com> (raw)
In-Reply-To: <CA+55aFwT3KFkGMgWz9Gag+xMHD=5X3RNz-iMswGD88_wnUT-Eg@mail.gmail.com>
On 10/03, Linus Torvalds wrote:
>
> On Fri, Oct 3, 2014 at 4:26 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > And I _think_ that preempt_schedule_context() should be fixed anyway,
> > although I am not sure there is no something else. It does:
> >
> >
> > preempt_disable_notrace();
> > prev_ctx = exception_enter();
> > preempt_enable_no_resched_notrace();
> >
> > preempt_schedule();
> >
> > preempt_disable_notrace();
> > exception_exit(prev_ctx);
> > preempt_enable_notrace();
> >
> > but exception_exit() is heavy, it is quite possible that TIF_NEED_RESCHED
> > and thus set_preempt_need_resched() can be set again when we call
> > preempt_enable_notrace(). And in this case preempt_schedule_context()
> > will be called recursively.
>
> Why the hell is it using "preempt_enable_notrace()" in the first
> place? Shouldn't it use "preempt_enable_no_resched_notrace()",
Yes, this this is the main problem.
> > Frederic, how about the patch below?
>
> Why do it multiple times? The whole concept is fundamentally racy
> anyway, in it doesn't guarantee that any *new* "need_resched()" would
> be reacted to (since they could happen *after* the test),
But in this case we rely on scheduler_ipi() and return-from-irq path?
> The real fix would appear to be to use
> "preempt_enable_no_resched_notrace()", which your patch did, but
> without the loop.
Not sure... preempt_schedule() does the same and afaics for good reason.
But perhaps you are right. I am already sleeping, will try to recheck
tomorrow. And in fact I got lost in preempt.h files... I can't even
understand why __preempt_schedule_context() is only called by
preempt_enable_notrace().
Oleg.
next prev parent reply other threads:[~2014-10-04 0:22 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-21 18:41 [PATCH v3 0/2] x86: reimplement ___preempt_schedule*() using THUNK helpers Oleg Nesterov
2014-09-21 18:41 ` [PATCH v3 1/2] " Oleg Nesterov
2014-09-24 15:02 ` [tip:x86/asm] x86: Speed up ___preempt_schedule*() by " tip-bot for Oleg Nesterov
2014-10-03 4:50 ` Sasha Levin
2014-10-03 13:39 ` Chuck Ebbert
2014-10-03 21:41 ` Oleg Nesterov
2014-10-03 21:56 ` Andy Lutomirski
2014-10-03 23:48 ` Linus Torvalds
2014-10-03 23:51 ` Oleg Nesterov
2014-10-03 22:48 ` Chuck Ebbert
2014-10-03 22:53 ` Andy Lutomirski
2014-10-03 23:13 ` H. Peter Anvin
2014-10-03 23:37 ` Oleg Nesterov
2014-10-03 21:16 ` Oleg Nesterov
2014-10-03 23:26 ` Oleg Nesterov
2014-10-04 0:01 ` Linus Torvalds
2014-10-04 0:11 ` Linus Torvalds
2014-10-04 0:33 ` Oleg Nesterov
2014-10-05 20:23 ` [PATCH 0/1] stop the unbound recursion in preempt_schedule_context() Oleg Nesterov
2014-10-05 20:23 ` [PATCH 1/1] " Oleg Nesterov
2014-10-28 11:03 ` [tip:sched/core] sched: " tip-bot for Oleg Nesterov
2014-10-05 23:53 ` [PATCH 0/1] " Oleg Nesterov
2014-10-04 0:19 ` Oleg Nesterov [this message]
2014-09-21 18:42 ` [PATCH v3 2/2] x86, lib/Makefile: remove the unnecessary "+= thunk_64.o" Oleg Nesterov
2014-09-24 15:02 ` [tip:x86/asm] x86/lib/Makefile: Remove " tip-bot for Oleg Nesterov
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