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Date: Thu, 3 Dec 2020 09:47:04 -0800 From: Linus Torvalds <torvalds@...ux-foundation.org> To: kernel test robot <oliver.sang@...el.com> Cc: David Howells <dhowells@...hat.com>, lkp@...ts.01.org, kernel test robot <lkp@...el.com>, "Huang, Ying" <ying.huang@...el.com>, Feng Tang <feng.tang@...el.com>, zhengjun.xing@...el.com, Pavel Begunkov <asml.silence@...il.com>, Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>, Alexander Viro <viro@...iv.linux.org.uk>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, linux-block <linux-block@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: [iov_iter] 9bd0e337c6: will-it-scale.per_process_ops -4.8% regression On Wed, Dec 2, 2020 at 10:31 PM kernel test robot <oliver.sang@...el.com> wrote: > > FYI, we noticed a -4.8% regression of will-it-scale.per_process_ops due to commit: Ok, I guess that's bigger than expected, but the profile data does show how bad the indirect branches are. There's both a "direct" cost of them: > 0.55 ą 14% +0.3 0.87 ą 15% perf-profile.children.cycles-pp.__x86_retpoline_rax > 0.12 ą 14% +0.1 0.19 ą 14% perf-profile.self.cycles-pp.__x86_indirect_thunk_rax > 0.43 ą 14% +0.3 0.68 ą 15% perf-profile.self.cycles-pp.__x86_retpoline_rax The actual retpoline profile costs themselves do not add up to 4%, but I think that's because the indirect costs are higher, because the branch mis-predicts will basically make everything run slower for a while as the OoO engine needs to restart. So the global cost then shows up in CPU and branch miss stats, where the IPC goes down (which is the same thing as saying that CPI goes up): > 1.741e+08 +42.3% 2.476e+08 perf-stat.i.branch-misses > 0.74 -3.9% 0.71 perf-stat.overall.ipc > 1.35 +4.1% 1.41 perf-stat.overall.cpi which is why it ends up being so costly even if the retpoline overhead itself is "only" just under 1%. Linus
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