linux-kernel - Re: + cpumask-dont-perform-while-loop-in-cpumask_next

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Message-ID: <20150615143859.GC538@swordfish>
Date:	Mon, 15 Jun 2015 23:38:59 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	akpm@...ux-foundation.org
Cc:	sergey.senozhatsky@...il.com, amirv@...lanox.com,
	davem@...emloft.net, tj@...nel.org, mm-commits@...r.kernel.org,
	peterz@...radead.org, bp@...en8.de, linux-kernel@...r.kernel.org,
	sergey.senozhatsky.work@...il.com
Subject: Re: + cpumask-dont-perform-while-loop-in-cpumask_next_and.patch
 added to -mm tree

On (03/02/15 15:58), akpm@...ux-foundation.org wrote:
> ------------------------------------------------------
> From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Subject: cpumask: don't perform while loop in cpumask_next_and()
> 
> cpumask_next_and() is looking for cpumask_next() in src1 in a loop and
> tests if found cpu is also present in src2. remove that loop, perform
> cpumask_and() of src1 and src2 first and use that new mask to find
> cpumask_next().
> 
> Apart from removing while loop, ./bloat-o-meter on x86_64 shows
> add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-8 (-8)
> function                                     old     new   delta
> cpumask_next_and                              62      54      -8
> 


Sir,

Please revert this commit.

It does awful things on really big machines (spotted by Peter Zijlstra).


	-ss

> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Amir Vadai <amirv@...lanox.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
> 
>  lib/cpumask.c |    9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff -puN lib/cpumask.c~cpumask-dont-perform-while-loop-in-cpumask_next_and lib/cpumask.c
> --- a/lib/cpumask.c~cpumask-dont-perform-while-loop-in-cpumask_next_and
> +++ a/lib/cpumask.c
> @@ -37,10 +37,11 @@ EXPORT_SYMBOL(__next_cpu_nr);
>  int cpumask_next_and(int n, const struct cpumask *src1p,
>  		     const struct cpumask *src2p)
>  {
> -	while ((n = cpumask_next(n, src1p)) < nr_cpu_ids)
> -		if (cpumask_test_cpu(n, src2p))
> -			break;
> -	return n;
> +	struct cpumask tmp;
> +
> +	if (cpumask_and(&tmp, src1p, src2p))
> +		return cpumask_next(n, &tmp);
> +	return nr_cpu_ids;
>  }
>  EXPORT_SYMBOL(cpumask_next_and);
>  
> _
> 
> Patches currently in -mm which might be from sergey.senozhatsky@...il.com are
> 
> cpumask-dont-perform-while-loop-in-cpumask_next_and.patch
> lib-lz4-pull-out-constant-tables.patch
> 
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