linux-kernel - Re: [PATCH] lib/percpu_counter.c: disable local irq when updating percpu couter

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Date:	Wed, 8 Jan 2014 11:29:32 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Shaohua Li <shli@...ionio.com>, Jens Axboe <axboe@...nel.dk>,
	Fan Du <fan.du@...driver.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] lib/percpu_counter.c: disable local irq when updating
 percpu couter

Hi Andrew,

On Wed, Jan 8, 2014 at 9:36 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
>> I am wondering if the above patch is more efficient, because:
>>
>> - raw_local_irq_save()/raw_local_irq_restore() should be cheaper
>> than preempt_enable() in theory
>
> Don't think so - local_irq_disable() requires quite some internal
> synchronization in the CPU and is expensive.  preempt_disable() is just

Yes, it might be a little expensive on some CPUs, but should be
arch-dependent(CPU inside things are involved)

> an add+barrier, minus the add if the kernel is non-preemptable.

IMO, generally, from software view, local_irq_save() only save the
CPU's interrupt mask to the local variable 'flag', and sets irq mask
to register, considered local variable can be thought to be in cache,
so I think it might be cheaper than preempt_enable() because
preempt counter may not be in cache.

Also this_cpu_add() won't work in batch path(slow path), we still
need to avoid interrupt coming between reading the percpu counter
and resetting it, otherwise counts might be lost too, :-)

Thanks,
-- 
Ming Lei
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