linux-kernel - Re: [PATCH] lib/spinlock_debug: Tweak the loop time to fit different

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Message-ID: <20140430095205.GF30445@twins.programming.kicks-ass.net>
Date:	Wed, 30 Apr 2014 11:52:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Liu, Chuansheng" <chuansheng.liu@...el.com>
Cc:	"Wang, Xiaoming" <xiaoming.wang@...el.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/spinlock_debug: Tweak the loop time to fit different
 _delay()

On Wed, Apr 30, 2014 at 09:30:55AM +0000, Liu, Chuansheng wrote:
> > From: Peter Zijlstra [mailto:peterz@...radead.org]
> > Sent: Wednesday, April 30, 2014 4:55 PM
> > To: Wang, Xiaoming
> > Cc: mingo@...hat.com; linux-kernel@...r.kernel.org; Liu, Chuansheng
> > Subject: Re: [PATCH] lib/spinlock_debug: Tweak the loop time to fit different
> > _delay()
> > 
> > On Wed, Apr 30, 2014 at 06:40:38PM -0400, Wang, Xiaoming wrote:
> > > loops_per_jiffy*Hz  is not always 1 second exactly
> > > it depends on the realization of _delay() .
> > > delay_tsc is used as _delay() in arch/x86/lib/delay.c
> > 
> > This just states delay() is broken. The primary response should be to
> > try and fix that, no?
> 
> 
> delay(1s_count) is accurate, but delay(1) is not accurate indeed, since executing
> some instruction, then the 1 cycle delay maybe be used already.

OK, so there's (finally) a problem statement, so is there anything sane
we can do about that?

But yes, a trylock is a cmpxchg, and a cmpxchg on a contended cacheline
can be _much_ longer than one loop.

Now the real problem is coming up with something that'll work for all
architectures.
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