Re: [RFC] sched, x86: Prevent resched interrupts if task in kernel mode and !CONFIG_PREEMPT

[Kirill Tkhai]

Ð ÐÐ, 26/01/2015 Ð 14:58 +0300, Kirill Tkhai ÐÐÑÐÑ:
> Ð ÐÑ, 23/01/2015 Ð 18:36 -0800, Andy Lutomirski ÐÐÑÐÑ:
> > On Fri, Jan 23, 2015 at 9:09 AM, Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> > > Ð ÐÑ, 23/01/2015 Ð 08:24 -0800, Andy Lutomirski ÐÐÑÐÑ:
> > >> On Fri, Jan 23, 2015 at 8:07 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >> > On Fri, Jan 23, 2015 at 06:53:32PM +0300, Kirill Tkhai wrote:
> > >> >> ---
> > >> >>  arch/x86/kernel/entry_64.S |   10 ++++++++++
> > >> >>  1 file changed, 10 insertions(+)
> > >> >>
> > >> >> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> > >> >> index c653dc4..a046ba8 100644
> > >> >> --- a/arch/x86/kernel/entry_64.S
> > >> >> +++ b/arch/x86/kernel/entry_64.S
> > >> >> @@ -409,6 +409,13 @@ GLOBAL(system_call_after_swapgs)
> > >> >>       movq_cfi rax,(ORIG_RAX-ARGOFFSET)
> > >> >>       movq  %rcx,RIP-ARGOFFSET(%rsp)
> > >> >>       CFI_REL_OFFSET rip,RIP-ARGOFFSET
> > >> >> +#if !defined(CONFIG_PREEMPT) || !defined(SMP)
> > >> >> +     /*
> > >> >> +      * Tell resched_curr() do not send useless interrupts to us.
> > >> >> +      * Kernel isn't preemptible till sysret_careful() anyway.
> > >> >> +      */
> > >> >> +     LOCK ; bts $TIF_POLLING_NRFLAG,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
> > >> >> +#endif
> > >>
> > >> That's kind of expensive.  What's the !SMP part for?
> > >
> > > smp_send_reschedule() is NOP on UP. There is no problem.
> > 
> > Shouldn't it be #if !defined(CONFIG_PREEMPT) && defined(CONFIG_SMP) then?
> 
> Definitely, thanks.
> 
> > 
> > >
> > >>
> > >> >>       testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
> > >> >>       jnz tracesys
> > >> >>  system_call_fastpath:
> > >> >> @@ -427,6 +434,9 @@ GLOBAL(system_call_after_swapgs)
> > >> >>   * Has incomplete stack frame and undefined top of stack.
> > >> >>   */
> > >> >>  ret_from_sys_call:
> > >> >> +#if !defined(CONFIG_PREEMPT) || !defined(SMP)
> > >> >> +     LOCK ; btr $TIF_POLLING_NRFLAG,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
> > >> >> +#endif
> > >>
> > >> If only it were this simple.  There are lots of ways out of syscalls,
> > >> and this is only one of them :(  If we did this, I'd rather do it
> > >> through the do_notify_resume mechanism or something.
> > >
> > > Yes, syscall is the only thing I did as an example.
> > >
> > >> I don't see any way to do this without at least one atomic op or
> > >> smp_mb per syscall, and that's kind of expensive.
> > >
> > > JFI, doesn't x86 set_bit() lock a small area of memory? I thought
> > > it's not very expensive on this arch (some bus optimizations or
> > > something like this).
> > 
> > An entire syscall on x86 is well under 200 cycles.  lock addl is >20
> > cycles for me, and I don't see why the atomic bitops would be faster.
> > (Oddly, mfence is slower than lock addl, which is really odd, since
> > lock addl implies mfence.)  So this overhead may actually matter.
> 
> Yeah, it's really big overhead.
> 
> > >
> > >> Would it make sense to try to use context tracking instead?  On
> > >> systems that use context tracking, syscalls are already expensive, and
> > >> we're already keeping track of which CPUs are in user mode.
> > >
> > > I'll look at context_tracking, but I'm not sure some smp synchronization
> > > there.
> > 
> > It could be combinable with existing synchronization there.
> 
> I'll look at this. Thanks!

Ðontinuing the theme. I've tried the idea with RCU. Fast & dirty patch
which prevents unnecessary interrupts.

It prevents 2% of reschedule IPIs. The cost is atomic_read() in
resched_curr(). It looks like the profit is not too much...

Kirill

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