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[209.85.167.42]) by smtp.gmail.com with ESMTPSA id o7sm3624817lff.73.2019.03.01.08.51.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 01 Mar 2019 08:51:54 -0800 (PST) Received: by mail-lf1-f42.google.com with SMTP id d26so1824409lfa.1 for ; Fri, 01 Mar 2019 08:51:54 -0800 (PST) X-Received: by 2002:ac2:415a:: with SMTP id c26mr3761301lfi.62.1551459114173; Fri, 01 Mar 2019 08:51:54 -0800 (PST) MIME-Version: 1.0 References: <20190228171242.32144-1-frederic@kernel.org> <20190301034536.GA19200@lenoir> In-Reply-To: <20190301034536.GA19200@lenoir> From: Linus Torvalds Date: Fri, 1 Mar 2019 08:51:38 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 00/37] softirq: Per vector masking v3 To: Frederic Weisbecker Cc: LKML , Sebastian Andrzej Siewior , Peter Zijlstra , "David S . Miller" , Mauro Carvalho Chehab , Thomas Gleixner , "Paul E . McKenney" , Frederic Weisbecker , Pavan Kondeti , Ingo Molnar , Joel Fernandes 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 Thu, Feb 28, 2019 at 7:45 PM Frederic Weisbecker wrote: > > Numbers are indeed missing. In fact this patchset mostly just brings an > infrastructure. We have yet to pinpoint the most latency-inducing > softirq disabled sites and make them disable only the vectors that > are involved in a given lock. Note that I think we pretty much know that already: the people who have had issues have never actually really had issues with the actual "disable softirq" paths, they've all been about actually *running* the softirq's (and that in turn being a latency issue for running _other_ softirqs, and in particular for delaying them into softirqd). Now, it may well be that yes, we'll have "block softirqs" code that has issues too, but it's absolutely been swamped by the latencies for actually running them so far. Note that this is all really fairly independent of the whole masking logic. Yes, the masking logic comes into play too (allowing you to run a subset of softirq's at a time), but on the whole the complaints I've seen have not been "the networking softirq takes so long that it delays USB tasklet handling", but they have been along the lines of "the networking softirq gets invoked so often that it then floods the system and triggers softirqd, and _that_ then makes tasklet handling latency go up insanely". See the difference? Not the latency of softirq's disabled, but the latency of one group of softirqs causing problems for another when they all get batched together (and soft-scheduled to another context together). Linus