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AJvYcCWMOffnvkod3azi4ClzqFqYsiR4bo0KyiXpChMOuf+jHeq20QL58O48Ado5eDdmmx/SseVbDWQL+e7c6r1WM+OUKec= X-Gm-Message-State: AOJu0Yx3KYk/h2KcGLGnRI6k+I5Yeia279WuSTrV9YNCIu16wUlZuz3T Depr+BC3L4KYrZoJ9Lo4ZLITdc5fY+JTFws4RYAHLmq3Yo7NE3UD8PTAo6UF465nq7Hj9we4i/J jmvOv23OSP3RcYrD+j1DuD+C7HoNxoAM7ojzCgA== X-Google-Smtp-Source: AGHT+IHLgPlajLx2pVxBiY+uW2kN+jbOqLvMD5j+Bx9XsZ8LoAl/WbjfKey1Lf18oYec9BuA1muX2WLIhYnEVfIYfSs= X-Received: by 2002:ac8:5cc3:0:b0:430:c1e4:2cb7 with SMTP id s3-20020ac85cc3000000b00430c1e42cb7mr8266936qta.43.1710781291061; Mon, 18 Mar 2024 10:01:31 -0700 (PDT) MIME-Version: 1.0 References: <20240311164638.2015063-1-pasha.tatashin@soleen.com> <2cb8f02d-f21e-45d2-afe2-d1c6225240f3@zytor.com> <2qp4uegb4kqkryihqyo6v3fzoc2nysuhltc535kxnh6ozpo5ni@isilzw7nth42> <39F17EC4-7844-4111-BF7D-FFC97B05D9FA@zytor.com> <74c82d8a6b5f490784cc8f16fa7d2c12@AcuMS.aculab.com> In-Reply-To: <74c82d8a6b5f490784cc8f16fa7d2c12@AcuMS.aculab.com> From: Pasha Tatashin Date: Mon, 18 Mar 2024 13:00:52 -0400 Message-ID: Subject: Re: [RFC 00/14] Dynamic Kernel Stacks To: David Laight Cc: "H. Peter Anvin" , Matthew Wilcox , Kent Overstreet , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "akpm@linux-foundation.org" , "x86@kernel.org" , "bp@alien8.de" , "brauner@kernel.org" , "bristot@redhat.com" , "bsegall@google.com" , "dave.hansen@linux.intel.com" , "dianders@chromium.org" , "dietmar.eggemann@arm.com" , "eric.devolder@oracle.com" , "hca@linux.ibm.com" , "hch@infradead.org" , "jacob.jun.pan@linux.intel.com" , "jgg@ziepe.ca" , "jpoimboe@kernel.org" , "jroedel@suse.de" , "juri.lelli@redhat.com" , "kinseyho@google.com" , "kirill.shutemov@linux.intel.com" , "lstoakes@gmail.com" , "luto@kernel.org" , "mgorman@suse.de" , "mic@digikod.net" , "michael.christie@oracle.com" , "mingo@redhat.com" , "mjguzik@gmail.com" , "mst@redhat.com" , "npiggin@gmail.com" , "peterz@infradead.org" , "pmladek@suse.com" , "rick.p.edgecombe@intel.com" , "rostedt@goodmis.org" , "surenb@google.com" , "tglx@linutronix.de" , "urezki@gmail.com" , "vincent.guittot@linaro.org" , "vschneid@redhat.com" Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 133B8140002 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: 7def18bks75a4t93sa17bfx4q3efw4d7 X-HE-Tag: 1710781291-779481 X-HE-Meta: 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 jwqfH8P+ P4BFKP7DFi0feO1jGcaYxXxeJNqORKGS2pzjm+Kb+9+pQB6JjeourwgT53lOgiGU1gtRh/4wUv7ZuqSD+J8VMinHBcGuxNTPfwEDrteT+bSNc70WXr9Tj2/CObWUIin4dgctOEM9o5QNWiAGz6NOg8rsB9XDAZX9+/bmSSrvY6Duz7ZOofqxVKwXVtpwonQlT1XjGiEM/5IuIEIO9wzwT2epKBfLKqsNutB7DsLEzB4SEqDbk3TddTZSEetk+eJmTvOF4WO8Xwcx5H4oPecE9QjWDurQkLP7Ie3kmaFX2z6laZCoc0YNL1lyEjpyOOBz8c0foSvqYlY9jNUhmpkUuM38vqEGtSGeAAVAFJ03PYIwEtZfpQ4lqYNG80QjN8oUsvV5C 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: List-Subscribe: List-Unsubscribe: On Mon, Mar 18, 2024 at 11:39=E2=80=AFAM David Laight wrote: > > ... > > - exit_to_user_mode(): Unmap the extra three pages and return them to > > the per-CPU cache. This function is called late in the kernel exit > > path. > > Why bother? > The number of tasks running in user_mode is limited to the number > of cpu. So the most you save is a few pages per cpu. > > Plausibly a context switch from an interrupt (eg timer tick) > could suspend a task without saving anything on its kernel stack. > But how common is that in reality? > In a well behaved system most user threads will be sleeping on > some event - so with an active kernel stack. > > I can also imagine that something like sys_epoll() actually > sleeps with not (that much) stack allocated. > But the calls into all the drivers to check the status > could easily go into another page. > You really wouldn't to keep allocating and deallocating > physical pages (which I'm sure has TLB flushing costs) > all the time for those processes. > > Perhaps a 'garbage collection' activity that reclaims stack > pages from processes that have been asleep 'for a while' or > haven't used a lot of stack recently (if hw 'page accessed' > bit can be used) might make more sense. > > Have you done any instrumentation to see which system calls > are actually using more than (say) 8k of stack? > And how often the user threads that make those calls do so? None of our syscalls, AFAIK. Pasha > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1= 1PT, UK > Registration No: 1397386 (Wales)