linux-kernel - Re: [PATCH v2 08/11] mm/vmstat: switch counter modification to cmpxchg

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Date:   Thu, 2 Mar 2023 11:47:35 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Christoph Lameter <cl@...ux.com>
Cc:     Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 08/11] mm/vmstat: switch counter modification to
 cmpxchg

On 09.02.23 16:01, Marcelo Tosatti wrote:
> In preparation to switch vmstat shepherd to flush
> per-CPU counters remotely, switch all functions that
> modify the counters to use cmpxchg.
> 
> To test the performance difference, a page allocator microbenchmark:
> https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/mm/bench/page_bench01.c
> with loops=1000000 was used, on Intel Core i7-11850H @ 2.50GHz.
> 
> For the single_page_alloc_free test, which does
> 
>          /** Loop to measure **/
>          for (i = 0; i < rec->loops; i++) {
>                  my_page = alloc_page(gfp_mask);
>                  if (unlikely(my_page == NULL))
>                          return 0;
>                  __free_page(my_page);
>          }
> 
> Unit is cycles.
> 
> Vanilla			Patched		Diff
> 159			165		3.7%
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> 
> Index: linux-vmstat-remote/mm/vmstat.c
> ===================================================================
> --- linux-vmstat-remote.orig/mm/vmstat.c
> +++ linux-vmstat-remote/mm/vmstat.c
> @@ -334,6 +334,188 @@ void set_pgdat_percpu_threshold(pg_data_
>   	}
>   }

I wonder why we get a diff that is rather hard to review because it 
removes all existing codes and replaces it by almost-identical code. Are 
you maybe moving a bunch of code while modifying some tiny bits at the 
same time?

-- 
Thanks,

David / dhildenb