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Date: Wed, 9 Jan 2019 10:52:26 -0800
From: Matthew Wilcox <willy@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH v2 0/4] /proc/stat: Reduce irqs counting performance
 overhead
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On Wed, Jan 09, 2019 at 01:37:23PM -0500, Waiman Long wrote:
> On 01/09/2019 01:24 PM, Matthew Wilcox wrote:
> > Did you even try just using the general purpose infrastructure that's
> > in place?  If that shows a performance problem _then_ it's time to make
> > this special snowflake just a little more special.  Not before.
> 
> I have looked into the percpu counter code. There are two aspects that I
> don't like to introduce to the interrupt handler's code path for
> updating the counts.
> 
> 1) There is a raw spinlock in the percpu_counter structure that may need
> to be acquired in the update path. This can be a performance drag
> especially if lockdep is enabled.
> 
> 2) The percpu_counter structure is 40 bytes in size on 64-bit systems
> compared with just 8 bytes for the percpu count pointer and an
> additional 4 bytes that I introduced in patch 2. With thousands of irq
> descriptors, it can consume quite a lot more memory. Memory consumption
> was a point that you brought up in one of your previous mails.

Then _argue that_.  Don't just go off and do something random without
explaining to the rest of us why we're wrong.