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

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Message-ID: <20171110080520.uslq75uucwlg5y7y@hirez.programming.kicks-ass.net>
Date:   Fri, 10 Nov 2017 09:05:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Xu <peterx@...hat.com>
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 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.