linux-kernel - Re: [PATCH] x86/perf/intel/cqm: Get rid of the silly for_each

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Message-ID: <alpine.DEB.2.11.1602182106140.2477@nanos>
Date:	Thu, 18 Feb 2016 21:15:05 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Vikas Shivappa <vikas.shivappa@...el.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
	Stephane Eranian <eranian@...gle.com>,
	Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [PATCH] x86/perf/intel/cqm: Get rid of the silly for_each_cpu
 lookups

On Wed, 17 Feb 2016, Thomas Gleixner wrote:
> On Wed, 17 Feb 2016, Vikas Shivappa wrote:
> 
> Please stop top posting, finally!
> 
> > But we have an extra static - static to avoid having it in the stack..
> 
> It's not about the cpu mask on the stack. The reason was that with cpumask off
> stack cpumask_and_mask() requires an allocation, which then can't be used in
> the starting/dying callbacks.
>  
> Darn, you are right to remind me.
> 
> Now, the proper solution for this stuff is to provide a library function as we
> need that for several drivers. No point to duplicate that functionality. I'll
> cook something up and repost the uncore/cqm set tomorrow.

Second thoughts on that.

cpumask_any_but() is fine as is, if we feed it topology_core_cpumask(cpu). The
worst case search is two bitmap_find_next() if the first search returned cpu.

Now cpumask_any_and() does a search as well, but the number of
bitmap_find_next() invocations is limited to the number of sockets if we feed
the cqm_cpu_mask as first argument. So for 4 or 8 sockets that's still a
reasonable limit. If the people with insane large machines care, we can
revisit that topic. It's still faster than for_each_online_cpu() :)

Thanks,

	tglx