linux-kernel - Re: [PATCH] locking/osq: Drop the overload of osq lock

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Message-ID: <20160625182839.GN30154@twins.programming.kicks-ass.net>
Date:	Sat, 25 Jun 2016 20:28:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...hat.com, dave@...olabs.net,
	will.deacon@....com, Waiman.Long@....com, benh@...nel.crashing.org
Subject: Re: [PATCH] locking/osq: Drop the overload of osq lock

On Sun, Jun 26, 2016 at 12:28:13AM +0800, Boqun Feng wrote:
> On Sat, Jun 25, 2016 at 06:09:22PM +0200, Peter Zijlstra wrote:
> > That works here, but it would not work for the need_resched() in
> > mutex_spin_on_owner() and mutex_optimistic_spin() which need equal
> > treatment.
> > 
> > Because those too we want to limit.
> > 
> > The count thing, while a little more cumbersome, is more widely
> > applicable than just the one OSQ case where we happen to have a cpu
> > number.
> > 
> 
> But if we don't have a cpu number, which vcpu's preemption are we
> trying to detect? 

_this_ vcpu's preemption. If the yield count of this cpu changes, we
know this vcpu has been scheduled and we should stop spinning.

This is similar to the need_resched() case, which check is _this_ cpu
should reschedule.