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[209.85.167.49]) by smtp.gmail.com with ESMTPSA id t144sm1877627lff.94.2020.03.26.09.58.01 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 26 Mar 2020 09:58:01 -0700 (PDT) Received: by mail-lf1-f49.google.com with SMTP id q5so5456425lfb.13 for ; Thu, 26 Mar 2020 09:58:01 -0700 (PDT) X-Received: by 2002:a05:6512:10cf:: with SMTP id k15mr6681163lfg.142.1585241881094; Thu, 26 Mar 2020 09:58:01 -0700 (PDT) MIME-Version: 1.0 References: <20200326055723.GL11705@shao2-debian> In-Reply-To: <20200326055723.GL11705@shao2-debian> From: Linus Torvalds Date: Thu, 26 Mar 2020 09:57:45 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [mm] fd4d9c7d0c: stress-ng.switch.ops_per_sec -30.5% regression To: kernel test robot Cc: Jann Horn , LKML , lkp@lists.01.org Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 25, 2020 at 10:57 PM kernel test robot wrote: > > FYI, we noticed a -30.5% regression of stress-ng.switch.ops_per_sec due to commit: > > commit: fd4d9c7d0c71866ec0c2825189ebd2ce35bd95b8 ("mm: slub: add missing TID bump in kmem_cache_alloc_bulk()") This looks odd. I would not expect the update of c->tid to have that noticeable an impact, even on a big machine that might be close to some scaling limit. It doesn't add any expensive atomic ops, and while it _could_ make a percpu cacheline dirty, I think that cacheline should already be dirty anyway under any load where this is noticeable. Plus this should be a relatively cold path anyway. So mind humoring me, and double-check that regression? Of course, it might be another "just magic cache placement" detail where code moved enough to make a difference. Or maybe it really ends up causing new tid mismatches and we end up failing the fast path in slub as a result. But looking at the stats that changed in your message doesn't make me go "yeah, that looks like a slub difference". So before we look more at this, I'd like to make sure that the regression is actually real, and not noise. Please? Linus