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Date: Wed, 7 Sep 2011 21:08:55 +1000
From: Anton Blanchard <anton@samba.org>
To: Theodore Tso <tytso@MIT.EDU>
Cc: Tejun Heo <tj@kernel.org>, Eric Dumazet <eric.dumazet@gmail.com>,
        adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] percpu_counter: Put a reasonable upper bound on
 percpu_counter_batch
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Hi Ted,

> Um, this was an ext4 patch and I pointed out it could cause
> problems.  (Specifically, data loss…)

I'm a bit confused. While the comment mentions ext4, the patch is just
putting an upper bound on the size of percpu_counter_batch and it is
useful for percpu_counter_compare() too:

 static void compute_batch_value(void)
 {
        int nr = num_online_cpus();

-       percpu_counter_batch = max(32, nr*2);
+       /*
+        * The cutoff point for the percpu_counter_compare() fast path grows
+        * at num_online_cpus^2 and on a big enough machine it will be
+        * unlikely to hit.
+        * We clamp the batch value to 1024 so the cutoff point only grows
+        * linearly past 512 CPUs.
+        */
+       percpu_counter_batch = clamp(nr*2, 32, 1024);
 }

The batch value should be opaque to the rest of the kernel. If ext4
requires a specific batch value we can use the functions that take
an explicit one (eg __percpu_counter_add).

Anton