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[209.85.208.179]) by smtp.gmail.com with ESMTPSA id x26-20020ac25dda000000b004fbc0c7218bsm1468322lfq.118.2023.09.11.14.16.36 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 11 Sep 2023 14:16:36 -0700 (PDT) Received: by mail-lj1-f179.google.com with SMTP id 38308e7fff4ca-2b703a0453fso83476481fa.3 for ; Mon, 11 Sep 2023 14:16:36 -0700 (PDT) X-Received: by 2002:a05:651c:203:b0:2bc:f439:b5a5 with SMTP id y3-20020a05651c020300b002bcf439b5a5mr9138299ljn.14.1694466995798; Mon, 11 Sep 2023 14:16:35 -0700 (PDT) MIME-Version: 1.0 References: <20230830184958.2333078-1-ankur.a.arora@oracle.com> <20230830184958.2333078-8-ankur.a.arora@oracle.com> <20230908070258.GA19320@noisy.programming.kicks-ass.net> <87zg1v3xxh.fsf@oracle.com> <87edj64rj1.fsf@oracle.com> <20230911124856.453fba22@gandalf.local.home> In-Reply-To: From: Linus Torvalds Date: Mon, 11 Sep 2023 14:16:18 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED To: Steven Rostedt Cc: Ankur Arora , Peter Zijlstra , linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, tglx@linutronix.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 5E64040017 X-Stat-Signature: oknkqhwtydop6atyszxrywhour1jkagr X-HE-Tag: 1694467001-443707 X-HE-Meta: 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 VzITV9BW Mvitv8Zwyp8zNZzRZ8dEdPrE7Nkvifqd1ZYWAcD2Hi1FsFbUDtUzuzbDuXyiu2jOZ54aOlH0vAW/IoPa91cwJWCZoGr/2ZayO32tKbN/Llw0/zqJ6Zytwcq+XB2Z8ol/ibAB9BqAAixtuhvS2Qdo98HORmLRL9IzkYu3FebwgMYHw6e/UiD9bl5Ko8ZEyzasaB2Y8XOJKomDZPQwy3uw0vTc8aHIqUOsazK04+ONp3l+SFuHLWX2U2/T/Scezz8oRfizGZUb5XUFiqFaSdKu3rATOTHdh2iUuK7FQLg7HHFayTkHHg/AAzwheK+rx074QUwAmSkTTypguVtqbPIWdQaT3pnH3O4bR2aRyu5vxuJk+5rWfaCBmFI8NGOYmC/jCT9Q7OHSI4puvXiRO6eLHV4b01MBkNlLcGyl+Ulat/itjSxOl+2R2s2WTWQn2vlY3RuhJlz+ejnIO9ey3f5hUr/UIi8dNIpN8mhkUzs1x9VB5x3K/BmY3GsQBv0Nhe0wqxCQUTzq3T9uE9f5JIHkXgirQGfiRrjAxsL6dhFc5pL+ILbkyXghHrtmGg/+9CWicVRoxS3O9U5x3r5o= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, 11 Sept 2023 at 13:50, Linus Torvalds wrote: > > Except we've actually been *adding* to this whole mess, rather than > removing it. So we have actively *expanded* on that preemption choice > with PREEMPT_DYNAMIC. Actually, that config option makes no sense. It makes the sched_cond() behavior conditional with a static call. But all the *real* overhead is still there and unconditional (ie all the preempt count updates and the "did it go down to zero and we need to check" code). That just seems stupid. It seems to have all the overhead of a preemptible kernel, just not doing the preemption. So I must be mis-reading this, or just missing something important. The real cost seems to be PREEMPT_BUILD -> PREEMPTION -> PREEMPT_COUNT and PREEMPT vs PREEMPT_DYNAMIC makes no difference to that, since both will end up with that, and thus both cases will have all the spinlock preempt count stuff. There must be some non-preempt_count cost that people worry about. Or maybe I'm just mis-reading the Kconfig stuff entirely. That's possible, because this seems *so* pointless to me. Somebody please hit me with a clue-bat to the noggin. Linus