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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
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