From: Peter Xu <peterx@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
Radim Kr??m???? <rkrcmar@redhat.com>,
Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] sched/swait: allow swake_up() to return
Date: Fri, 10 Nov 2017 15:10:17 +0800 [thread overview]
Message-ID: <20171110071017.GH20627@xz-mi> (raw)
In-Reply-To: <20171109102303.vsetymxc6rllomhh@hirez.programming.kicks-ass.net>
On Thu, Nov 09, 2017 at 11:23:03AM +0100, Peter Zijlstra wrote:
> On Thu, Nov 09, 2017 at 05:18:53PM +0800, Peter Xu wrote:
> > Let swake_up() to return whether any of the waiters is waked up. One use
> > case of it would be:
> >
> > if (swait_active(wq)) {
> > swake_up(wq);
> > // do something when waiter is waked up
> > waked_up++;
> > }
>
> The word is 'woken', and no that doesn't work. All it says is that there
> was a waiter, not that you were to one to wake it. Another concurrent
> wakeup might have done so.
Yes. Or IIUC the waiter can be calling finish_swait() somehow so it
removed itself from the list before being woken.
>
> >
> > Logically it's possible that when reaching swake_up() the wait queue is
> > not active any more, and here doing something like waked_up++ would be
> > inaccurate. To correct it, we need an atomic version of it.
> >
> > With this patch, we can simply re-write it into:
> >
> > if (swake_up(wq)) {
> > // do something when waiter is waked up
> > waked_up++;
> > }
> >
> > After all we are checking swait_active() inside swake_up() too.
>
> We're not in fact; you've been staring at old code; see commit:
>
> 35a2897c2a30 ("sched/wait: Remove the lockless swait_active() check in swake_up*()")
I thought the tree was new enough, but obviously I was wrong...
Thanks for the pointer.
>
>
> Also, you're changing the interface relative to the regular wait
> interface. The two should be similar wherever possible.
Indeed.
I came to this when reading kvm_vcpu_wake_up(), so that only affects
some statistic which may not be that critical. However I don't know
whether there would be any other real use case that we would like to
know exactly whether a call to [s]wake_up() has really done something
or just returned with a NOP.
Anyway, please let me know if you think the same change to wake_up()
would be meaningful, otherwise I can drop this patch and post another
KVM-only one to clean up the redundant callers of swait_active(),
since even if we dropped that list check in 35a2897c2a30, we'll do
that again in swake_up_locked().
And after knowing 35a2897c2a30, I do think that calling swait_active()
before swake_up() is not good since that call is without a lock as
well, just like what can happen before 35a2897c2a30.
(I am not 100% sure whether I fully understand the problem mentioned
in 35a2897c2a30, but I think it's the memory barrier in the
lock/unlock that matters.)
Thanks,
--
Peter Xu
next prev parent reply other threads:[~2017-11-10 7:10 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20171109091854.24367-1-peterx@redhat.com>
2017-11-09 9:18 ` [PATCH 1/2] sched/swait: allow swake_up() to return Peter Xu
2017-11-09 10:06 ` Paolo Bonzini
2017-11-10 7:12 ` Peter Xu
2017-11-09 10:23 ` Peter Zijlstra
2017-11-10 7:10 ` Peter Xu [this message]
2017-11-10 8:05 ` Peter Zijlstra
2017-11-13 3:33 ` Peter Xu
2017-11-13 5:19 ` Peter Xu
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