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Date: Thu, 17 Oct 2013 11:26:23 -0700 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Peter Zijlstra <peterz@...radead.org> Cc: Don Zickus <dzickus@...hat.com>, Andi Kleen <ak@...ux.intel.com>, dave.hansen@...ux.intel.com, Stephane Eranian <eranian@...gle.com>, jmario@...hat.com, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Arnaldo Carvalho de Melo <acme@...radead.org>, Ingo Molnar <mingo@...nel.org> Subject: Re: [PATCH] perf, x86: Optimize intel_pmu_pebs_fixup_ip() On Thu, Oct 17, 2013 at 9:30 AM, Peter Zijlstra <peterz@...radead.org> wrote: > > So avoid having to call copy_from_user_nmi() for every instruction. > Since we already limit the max basic block size, we can easily > pre-allocate a piece of memory to copy the entire thing into in one > go. copy_from_user_nmi() itself is all kinds of nasty. Using __get_user_pages_fast() for a single page is quite expensive, and mucks around with the page counts etc. If copy_from_user_nmi() just did the (simple) page table walk by hand, it could avoid *all* of that. No page count stuff - just have interrupts disabled over not just the page walk, but the copy too - to guarantee that no cross-CPU TLB flush can come in. So instead of trying to improve __get_user_pages_fast() - which is impossible because the interface fundamentally means that it has to iterate over things and check page counts - you could simplify the caller instead. That is, if we really care any more. Maybe this "do the copy_from_user_nmi() just once" is already good enough that nobody much cares. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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