From mboxrd@z Thu Jan 1 00:00:00 1970 From: Florian Weimer Subject: Re: [PATCH] sched/core: expand sched_getaffinity(2) to return number of CPUs Date: Mon, 08 Apr 2019 09:49:35 +0200 Message-ID: <878swlszqo.fsf@oldenburg2.str.redhat.com> References: <20190403200809.GA13876@avx2> <20190404084249.GS4038@hirez.programming.kicks-ass.net> <87wok83gfs.fsf@oldenburg2.str.redhat.com> <20190406194825.GA5106@avx2> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <20190406194825.GA5106@avx2> (Alexey Dobriyan's message of "Sat, 6 Apr 2019 22:48:25 +0300") Sender: linux-kernel-owner@vger.kernel.org To: Alexey Dobriyan Cc: Peter Zijlstra , mingo@redhat.com, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org List-Id: linux-api@vger.kernel.org * Alexey Dobriyan: >> >> Patch overloads sched_getaffinity(len=0) to simply return "nr_cpu_ids". >> >> This will make gettting CPU mask require at most 2 system calls >> >> and will eliminate unnecessary code. >> >> >> >> len=0 is chosen so that >> >> * passing zeroes is the simplest thing >> >> >> >> syscall(__NR_sched_getaffinity, 0, 0, NULL) >> >> >> >> will simply do the right thing, >> >> >> >> * old kernels returned -EINVAL unconditionally. >> >> >> >> Note: glibc segfaults upon exiting from system call because it tries to >> >> clear the rest of the buffer if return value is positive, so >> >> applications will have to use syscall(3). >> >> Good news is that it proves noone uses sched_getaffinity(pid, 0, NULL). >> >> Given that old kernels fail with EINVAL, that evidence is fairly >> restricted. >> >> I'm not sure if it's a good idea to overload this interface. I expect >> that users will want to call sched_getaffinity (the system call wrapper) >> with cpusetsize == 0 to query the value, so there will be pressure on >> glibc to remove the memset. At that point we have an API that obscurely >> fails with old glibc versions, but suceeds with newer ones, which isn't >> great. > > I can do "if (len == 536870912)" so that bit count overflows on old > kernels into EINVAL and is unlikely to be used ever. I don't see how this solves this particular issue. It will still result in a mysterious crash if programs use an updated system call wrapper. Thanks, Florian