From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.3 required=5.0 tests=MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL,SPF_PASS autolearn=ham autolearn_force=no version=3.4.2 Received: (qmail 12600 invoked from network); 15 Apr 2020 15:58:28 -0000 Received-SPF: pass (mother.openwall.net: domain of lists.openwall.com designates 195.42.179.200 as permitted sender) receiver=inbox.vuxu.org; client-ip=195.42.179.200 envelope-from= Received: from mother.openwall.net (195.42.179.200) by inbox.vuxu.org with UTF8ESMTPZ; 15 Apr 2020 15:58:28 -0000 Received: (qmail 3406 invoked by uid 550); 15 Apr 2020 15:58:25 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 3383 invoked from network); 15 Apr 2020 15:58:24 -0000 Date: Wed, 15 Apr 2020 11:58:11 -0400 From: Rich Felker To: Florian Weimer Cc: Norbert Lange , musl@lists.openwall.com Message-ID: <20200415155811.GD11469@brightrain.aerifal.cx> References: <20200409181824.GD13749@port70.net> <87r1wwik8t.fsf@mid.deneb.enyo.de> <20200410010255.GN11469@brightrain.aerifal.cx> <877dyinzuz.fsf@mid.deneb.enyo.de> <20200414155522.GB11469@brightrain.aerifal.cx> <87wo6iknx1.fsf@mid.deneb.enyo.de> <87o8rtjcwj.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87o8rtjcwj.fsf@mid.deneb.enyo.de> User-Agent: Mutt/1.5.21 (2010-09-15) Subject: Re: [musl] [BUG] sysconf implementing _SC_NPROCESSORS_(CONF|ONLN) incorrectly On Wed, Apr 15, 2020 at 11:50:36AM +0200, Florian Weimer wrote: > * Norbert Lange: > > > How should one deal with this? > > I understand that the semantics are vague, but given that musl now > > implements this > > function, it will make detection and fallback hard (especially as musl > > doesn't wants to be identified by the likes of macros). > > > > As it is now, just using the affinity mask definitely cant be useful, > > an application wanting that behavior should be patched to > > use that function directly. > > If musl would not define the _SC_NPROCESSORS_* macros (but still keep > > the implementation), > > this could be used for compile-time detection atleast. Enabling the > > current implementation would be > > just a matter of explicitly defining those macros. > > _SC_NPROCESSORS_* as implemented in glibc is bad because those values > are not adjusted by cgroups, so it can grossly overestimate available > resources. > > The cgroups interfaces themselves are not stable and very complicated. > I don't think it's a good idea to target them, especially not from > code that is expected to be linked statically into applications. > > Given that, I'm not sure that glibc's way is a significant > improvement. musl should perhaps be changed to cope more gracefully > with a sched_getaffinity failure, though (by not reporting a UP > environment by accident). For what it's worth, even without the sched_getaffinity failure, it's still problematic for programs linked to musl to be using the values obtained to omit memory barriers since they may be restricted to a single core themselves but communicating over shared memory with another process that's not restricted or restricted to a different core. There really should be some documented meaning for the return values, whereby we decide either that such sketchy application usage is supported (e.g. document that values less than 2 are never returned, so that applications doing the hack always use barriers and they have no remaining documented way to determine it's really a UP environment) or declare the application usage incorrect/buggy (i.e. that the values may be specific to the cgroup or other resource-constraints (possibly virtualized) and can't be relied on if you're communicating with processes that might live outside those resource constraints). Rich