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YW5jZSB3aXRob3V0IHJldHBvbGluZSAtIEkgYW0gcHJvYmFibHkgd3JvbmcuDQoNCk90aGVyd2lz ZSwgSSBkbyBub3Qgc2VlIHdoeSB0aGlzIHBhdGNoIHNob3VsZCBiZSByZW1vdmVkLiBXZSBjYW4N Cmp1c3QgYXMgd2VsbCBhZGQgYSBidXN5LXdhaXQgbG9vcCB0byB0bGJfaXNfbm90X2xhenkoKSB0 byBnZXQgdGhlDQpzYW1lIGVmZmVjdOKApg0KDQo= From mboxrd@z Thu Jan 1 00:00:00 1970 Content-Type: multipart/mixed; boundary="===============2533587953761176117==" MIME-Version: 1.0 From: Nadav Amit To: lkp@lists.01.org Subject: Re: [x86/mm/tlb] 6035152d8e: will-it-scale.per_thread_ops -13.2% regression Date: Thu, 17 Mar 2022 19:02:32 +0000 Message-ID: <3B958B13-75F0-4B81-B8CF-99CD140436EB@vmware.com> In-Reply-To: List-Id: --===============2533587953761176117== Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable > On Mar 17, 2022, at 11:38 AM, Dave Hansen wrote: > = > On 3/17/22 02:04, kernel test robot wrote: >> FYI, we noticed a -13.2% regression of will-it-scale.per_thread_ops due = to commit: > ... >> commit: 6035152d8eebe16a5bb60398d3e05dc7799067b0 ("x86/mm/tlb: Open-code= on_each_cpu_cond_mask() for tlb_is_not_lazy()") >> https://nam04.safelinks.protection.outlook.com/?url=3Dhttps%3A%2F%2Fgit.= kernel.org%2Fcgit%2Flinux%2Fkernel%2Fgit%2Ftorvalds%2Flinux.git&data=3D= 04%7C01%7Cnamit%40vmware.com%7Cc958c9b39db94b6b78bc08da084564df%7Cb39138ca3= cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637831391403951050%7CUnknown%7CTWFpbGZsb3= d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000= &sdata=3DNSCrUK12AX5BtehaNZmPXIpirVtDOrllgUct1mqglO8%3D&reserved=3D= 0 master > ... >> 24.77 =C2=B1 2% +8.1 32.86 =C2=B1 3% perf-profile.self= .cycles-pp.llist_add_batch > = > = > tl;dr: This commit made the tlb_is_not_lazy() check happen earlier. > That earlier check can miss threads _going_ lazy because if mmap_lock > contention. Fewer lazy threads means more IPIs and lower performance. > = > =3D=3D=3D > = > There's a lot of noise in that profile, but I filtered most of it out. > The main thing is that, somehow the llist_add() in > smp_call_function_many_cond() got more expensive. Either we're doing > more of them or the cacheline is bouncing around more. > = > Turns out that we're sending *more* IPIs with this patch applied than > without. That shouldn't happen since the old code did the same exact > logical check: > = > if (cond_func && !cond_func(cpu, info)) > continue; > = > and the new code does: > = > if (tlb_is_not_lazy(cpu)) > ... > = > where cond_func=3D=3Dtlb_is_not_lazy. > = > So, what's the difference? Timing. With the old scheme, if a CPU > enters lazy mode between native_flush_tlb_others() and > the loop in smp_call_function_many_cond(), it won't get an IPI and won't > need to do the llist_add(). > = > I stuck some printk()s in there and can confirm that the > earlier-calculated mask always seems to have more bits set, at least > when running will-it-scale tests that induce TLB flush IPIs. > = > I was kinda surprised that there were so many threads going idle with a > cpu-eating micro like this. But, it makes sense since they're > contending on mmap_lock. Basically, since TLB-flushing operations like > mmap() hold mmap_lock for write they tend to *force* other threads into > idle. Idle threads are lazy and they tend to _become_ lazy around the > time that the flushing starts. > = > This new "early lazy check" behavior could theoretically work both ways. > If threads tended to be waking up from idle when TLB flushes were being > sent, this would tend to reduce the number of IPIs. But, since they > tend to be going to sleep it increases the number of IPIs. > = > Anybody have a better theory? I think we should probably revert the comm= it. Let=E2=80=99s get back to the motivation behind this patch. Originally we had an indirect branch that on system which are vulnerable to Spectre v2 translates into a retpoline. So I would not paraphrase this patch purpose as =E2=80=9Cearly lazy check= =E2=80=9D but instead =E2=80=9Cmore efficient lazy check=E2=80=9D. There is very litt= le code that was executed between the call to on_each_cpu_cond_mask() and the actual check of tlb_is_not_lazy(). So what it seems to happen in this test-case - according to what you say - is that *slower* checks of is-lazy allows to send fewer IPIs since some cores go into idle-state. Was this test run with retpolines? If there is a difference in performance without retpoline - I am probably wrong. Otherwise, I do not see why this patch should be removed. We can just as well add a busy-wait loop to tlb_is_not_lazy() to get the same effect=E2=80=A6 --===============2533587953761176117==--