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[46.139.12.213]) by smtp.gmail.com with ESMTPSA id o13-v6sm10272177wmc.8.2018.07.15.18.04.59 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sun, 15 Jul 2018 18:05:00 -0700 (PDT) Date: Mon, 16 Jul 2018 03:04:57 +0200 From: Ingo Molnar To: Rik van Riel Cc: linux-kernel@vger.kernel.org, x86@kernel.org, luto@kernel.org, dave.hansen@intel.com, kernel-team@fb.com, efault@gmx.de, tglx@linutronix.de, songliubraving@fb.com, hpa@zytor.com Subject: Re: [PATCH 7/7] x86,switch_mm: skip atomic operations for init_mm Message-ID: <20180716010457.GA28744@gmail.com> References: <20180710142833.26231-1-riel@surriel.com> <20180710142833.26231-8-riel@surriel.com> <20180715230430.GD30102@gmail.com> <1531698583.26425.3.camel@surriel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1531698583.26425.3.camel@surriel.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Rik van Riel wrote: > On Mon, 2018-07-16 at 01:04 +0200, Ingo Molnar wrote: > > * Rik van Riel wrote: > > > > > + /* > > > + * Stop remote flushes for the previous mm. > > > + * Skip the idle task; we never send init_mm TLB > > > flushing IPIs, > > > + * but the bitmap manipulation can cause cache > > > line contention. > > > + */ > > > + if (real_prev != &init_mm) { > > > + VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu, > > > + mm_cpumask(real_pr > > > ev))); > > > + cpumask_clear_cpu(cpu, > > > mm_cpumask(real_prev)); > > > > BTW., could this optimization be (safely) extended to all (or most) > > !task->mm > > kernel threads? > > > > In particular softirq and threaded irq handlers could benefit greatly > > I suspect in > > certain networking intense workloads that happen to active them. > > Yes, it could. > > Are there kernel threads that use something other than > init_mm today? Yeah, I think that's the typical case - so at minimum the comment should be fixed: > > > + * Skip the idle task; we never send init_mm TLB flushing IPIs, and it should say 'skip kernel threads', right? Thanks, Ingo