On Tue, Mar 12, 2019 at 9:59 AM Marc-André Lureau <marcandre.lur...@gmail.com> wrote: > > Hi > > On Fri, Mar 1, 2019 at 12:13 PM Mathias Fröhlich > <mathias.froehl...@gmx.net> wrote: > > > > On Friday, 1 March 2019 12:15:08 CET Eero Tamminen wrote: > > > Hi, > > > > > > On 1.3.2019 11.12, Michel Dänzer wrote: > > > > On 2019-02-28 8:41 p.m., Marek Olšák wrote: > > > >>> On Thu, Feb 28, 2019 at 1:37 PM Eero Tamminen > > > >>> <eero.t.tammi...@intel.com> > > > >>>> Why distro versions of Qemu filter sched_setaffinity() syscall? > > > >>> > > > >>> (https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1815889) > > > >>> > > > >>> Daniel Berrange (berrange) wrote on 2019-02-27: #19 > > > >>> > > > >>> "IMHO that mesa change is not valid. It is settings its affinity to > > > >>> run on all threads which is definitely *NOT* something we want to be > > > >>> allowed. Management applications want to control which CPUs QEMU runs > > > >>> on, and as such Mesa should honour the CPU placement that the QEMU > > > >>> process has. > > > >>> > > > >>> This is a great example of why QEMU wants to use seccomp to block > > > >>> affinity changes to prevent something silently trying to use more CPUs > > > >>> than are assigned to this QEMU." > > > >>> > > > >> > > > >> Mesa uses thread affinity to optimize memory access performance on some > > > >> CPUs (see util_pin_thread_to_L3). Other places in Mesa need to restore > > > >> the > > > >> original thread affinity for some child threads. Additionally, if games > > > >> limit the thread affinity, Mesa needs to restore the full thread > > > >> affinity > > > >> for some of its child threads. > > > > > > > > The last part sounds like Mesa clearly overstepping its authority. > > > > > > > > > > > >> In essence, the thread affinity should only be considered a hint for > > > >> the > > > >> kernel for optimal performance. There is no reason to kill the process > > > >> if > > > >> it's disallowed. Just ignore the call or modify the thread mask to > > > >> make it > > > >> legal. > > > > > > > > The fundamental issue here is that Mesa is using the thread affinity API > > > > for something else than it's intended for. If there was an API for what > > > > Mesa wants (encouraging certain sets of threads to run on topologically > > > > close cores), there should be no need to block that. > > > > > > Why such process needs to be killed instead the request being masked > > > suitably, is there some program that breaks subtly if affinity request > > > is masked (and that being worse than the program being killed)? > > > > But that is still a situation that could be nicely handled with a > > EPERM error return. Way better than just kill a process. > > That 'badly affected' program still can call abort then. > > But nicely working programs don't get just killed then!! > > > Returning an error seems less secure that prohibiting it completely. > And it may lead to subtle bugs in rarely tested code paths. > > It's legitimate that QEMU and management layers want to prevent > arbitrary code from changing resource allocation etc. > > There are no easy way I can think of for mesa (and other libraries) to > probe the seccomp filters and associated action. > > So we need a way to tell mesa not to call setaffinity() (and other > syscalls). MESA_NO_THREAD_AFFINITY or MESA_NO_SYSCALLS=setaffinity,... > seem like a relatively easy way to go.
I strongly believe we should not be going the route of adding another environment variable. Primarily because this is adding a big pitfall for users that have their software not work and have to spend significant efforts to figure out the environment variable and get things work. (as well as associated costs of some users not getting that far and filing bugs or proclaiming on other sites that this thing is buggy for them). As such I'd strongly appreciate it if people look further than the immediate crash and figure out a way to make graceful degradation happen without user intervention. Is there really no way to figure out that calling setaffinity is going to kill the process, and what would be needed to add a method? > > thanks > > > -- > Marc-André Lureau > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/mesa-dev _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
