linux-kernel - Re: [PATCH 1/2] sched/swait: allow swake

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Date:   Mon, 13 Nov 2017 11:33:43 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Kr??m???? <rkrcmar@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/swait: allow swake_up() to return

On Fri, Nov 10, 2017 at 09:05:20AM +0100, Peter Zijlstra wrote:
> On Fri, Nov 10, 2017 at 03:10:17PM +0800, Peter Xu wrote:
> > 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().
> 
> See commits:
> 
>   8cd641e3c7cb ("sched/wait: Add swq_has_sleeper()")
>   5e0018b3e39e ("kvm: Serialize wq active checks in kvm_vcpu_wake_up()")
> 
> 
> In any case, I don't think we want the change you propose. The numbers
> don't mean much and there's no point in making all the callers in the
> kernel slower for it.

I see. And also we can introduce a new API for that if really needed.

I'll repost with KVM only changes.  Thanks for reviewing.

-- 
Peter Xu