Messages in this thread | | | From | Paul Turner <> | Date | Tue, 27 Oct 2015 22:19:24 -0700 | Subject | Re: [RFC PATCH v2 2/3] restartable sequences: x86 ABI |
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On Tue, Oct 27, 2015 at 10:03 PM, Peter Zijlstra <peterz@infradead.org> wrote: > > On Tue, Oct 27, 2015 at 04:57:05PM -0700, Paul Turner wrote: > > +static void rseq_sched_out(struct preempt_notifier *pn, > > + struct task_struct *next) > > +{ > > + set_thread_flag(TIF_NOTIFY_RESUME); > > +} > > > > static __read_mostly struct preempt_ops rseq_preempt_ops = { > > .sched_in = rseq_sched_in_nop, > > - .sched_out = rseq_sched_out_nop, > > + .sched_out = rseq_sched_out, > > }; > > Since we're unconditionally setting this TIF flag for these tasks, can't > we introduce something similar to the (contested) TIF_NOHZ_FULL thing > which is kept on the task indefinitely. > So Andy and I talked about this also, I'm in favor, in particular this has two nice effects: a) In exit_to_usermode_loop() we can ensure that this is evaluated prior to _TIF_SIGPENDING. This removes the current requirement that we also validate this state in setup_rt_frame() [which can perturb this state prior to our existing notifier]. b) We avoid spurious interactions with other things that use notify resume.
> That avoids having the preempt notifiers and this atomic op in the > schedule path.
So we still want something there (although it can be definitely be inlined as opposed to a preempt_notifier) since this allows us to only evaluate this check on returns to user-space that might matter as opposed to every syscall.
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