linux-ext4 - Re: [PATCH] percpu_counter: Fix __percpu_counter

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Date:	Fri, 12 Dec 2008 18:47:05 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Theodore Tso <tytso@....edu>,
	linux kernel <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Mingming Cao <cmm@...ibm.com>, linux-ext4@...r.kernel.org,
	Christoph Lameter <clameter@....com>
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

On Thursday 11 December 2008 09:26:37 Eric Dumazet wrote:
> But then, some (all but x86 ;) ) arches dont have true local_t and we fallback
> to plain atomic_long_t, and this is wrong because it would add a LOCKED
> instruction in fast path.
> 
> I remember Christoph added FAST_CMPXCHG_LOCAL, but no more uses of it in current
> tree.
> 
> Ie : using local_t only if CONFIG_FAST_CMPXCHG_LOCAL, else something like :
> 
> void __percpu_counter_add_irqsafe(struct percpu_counter *fbc, s64 amount, s32 batch)
> {
>         s64 count;
>         s32 *pcount = per_cpu_ptr(fbc->counters, get_cpu());
>         unsigned long flags;
> 
>         local_irq_save(flags);
>         count = *pcount + amount;

This is dumb though.  If local_irq_save(), add, local_irq_restore() is faster
than atomic_long_add on some arch, *that* is what that arch's local_add()
should do!

Open coding it like this is obviously wrong.

Now, archs local.h need attention (x86-32 can be optimized today, for
example), but that's not directly related.

Hope that clarifies,
Rusty.
PS.  Yes, I should produce a documentation patch and fix the x86 version.
Added to TODO list.
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