From: Oleg Nesterov <oleg@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>
Cc: Sasha Levin <sasha.levin@oracle.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: [PATCH 0/1] stop the unbound recursion in preempt_schedule_context()
Date: Sun, 5 Oct 2014 22:23:00 +0200 [thread overview]
Message-ID: <20141005202300.GA27962@redhat.com> (raw)
In-Reply-To: <20141004003300.GA6297@redhat.com>
On 10/04, Oleg Nesterov wrote:
>
> On 10/03, Linus Torvalds wrote:
> >
> > On Fri, Oct 3, 2014 at 5:01 PM, Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > The real fix would appear to be to use
> > > "preempt_enable_no_resched_notrace()", which your patch did, but
> > > without the loop.
> >
> > Actually, the real fix would be to not be stupid, and just make the
> > code do something like
> >
> > > if (likely(!preemptible()))
> > > return;
> > >
> > > __preempt_count_add(PREEMPT_ACTIVE);
> > > prev_ctx = exception_enter();
> > >
> > > __schedule();
> > >
> > > exception_exit(prev_ctx);
> > > __preempt_count_sub(PREEMPT_ACTIVE);
> >
> > and *not* enable preemption around the scheduling at all.
Yes, I think you are right. And I hate to admit that I didn't think about
this simplification. The only complication is that we should move this
function from context_tracking.c to sched/core.c.
However, I still think we need the need_resched() loop, we can't do this
only once.
And in fact the simplest fix could just turn it into
local_irq_disable();
preempt_schedule_irq();
local_irq_enable();
> Again, it is too late for me... Most probably I am wrong, but somehow
> it seems to me that the real fix should try to kill preempt_schedule_context()
> altogether and teach preempt_schedule() to play well with CONTEXT_TRACKING.
Yes, the very fact that
preempt_enable() != trace_preempt_on() + preempt_enable_notrace()
looks simply wrong imo. And preempt_enable_notrace() has users outside of
the tracing code which can run in IN_USER state. For example, why should
__perf_sw_event() worry about context_tracking.state?
OTOH, if the caller of preempt_enable_notrace() actually needs to take care
of potential IN_USER state, then it probably has other problems. And indeed,
ftrace_ops_control_func() has to check rcu_is_watching() anyway.
IOW, imho 29bb9e5a75 "tracing/context-tracking: Add preempt_schedule_context()
for tracing" was not a right solution. Perhaps the tracing functions can be
changed to switch to IN_KERNEL mode, or at least perhaps we can add the
special preempt_disable_ftrace() helper. But this needs another discussion.
Either way, I do think that preempt_schedule_context() in its current form
must die.
Oleg.
next prev parent reply other threads:[~2014-10-05 20:26 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 ` Oleg Nesterov [this message]
2014-10-05 20:23 ` [PATCH 1/1] stop the unbound recursion in preempt_schedule_context() 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 ` [tip:x86/asm] x86: Speed up ___preempt_schedule*() by using THUNK helpers Oleg Nesterov
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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