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Date: Thu, 21 Mar 2019 10:12:35 -0700 From: hpa@...or.com To: Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org> CC: Thomas Gleixner <tglx@...utronix.de>, Julien Thierry <julien.thierry@....com>, Will Deacon <will.deacon@....com>, Andy Lutomirski <luto@...capital.net>, Ingo Molnar <mingo@...nel.org>, Catalin Marinas <catalin.marinas@....com>, James Morse <james.morse@....com>, valentin.schneider@....com, Brian Gerst <brgerst@...il.com>, Josh Poimboeuf <jpoimboe@...hat.com>, Andrew Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>, Denys Vlasenko <dvlasenk@...hat.com>, Linux List Kernel Mailing <linux-kernel@...r.kernel.org>, Dmitry Vyukov <dvyukov@...gle.com>, Steven Rostedt <rostedt@...dmis.org> Subject: Re: [PATCH 01/25] x86: Make SMAP 64-bit only On March 18, 2019 11:10:22 AM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote: >On Mon, Mar 18, 2019 at 10:51 AM Peter Zijlstra <peterz@...radead.org> >wrote: >> >> How about I do a patch that schedules EFLAGS for both 32bit and >64bit, >> mark this for backporting to infinity. >> >> And then at the end, after the objtool-ac bits land, I do a patch >> removing the EFLAGS scheduling for x86_64. > >Sounds sane to me. > >And we can make it AC-conditional if it's actually shown to be visible >from a performance standpoint. > >But iirc pushf/popf isn't really that expensive - in fact I think it's >pretty cheap when system flags don't change. Which would be the common >case unless you'd also make the popf do the irq restore part and >simply make finish_lock_switch() re-enable irq's by doing an >irqrestore? > >I think popf is like 20 cycles or something (and pushf is just single >cycles). Probably not worth worrying about in the task switch context. > > Linus Yes, the problem isn't scheduling per se but the risk of hiding problems. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
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