Messages in this thread | | | Subject | Re: [RFC] sched, x86: Prevent resched interrupts if task in kernel mode and !CONFIG_PREEMPT | From | Kirill Tkhai <> | Date | Tue, 3 Feb 2015 20:14:53 +0300 |
| |
В Пн, 26/01/2015 в 14:58 +0300, Kirill Tkhai пишет: > В Пт, 23/01/2015 в 18:36 -0800, Andy Lutomirski пишет: > > On Fri, Jan 23, 2015 at 9:09 AM, Kirill Tkhai <ktkhai@parallels.com> wrote: > > > В Пт, 23/01/2015 в 08:24 -0800, Andy Lutomirski пишет: > > >> On Fri, Jan 23, 2015 at 8:07 AM, Peter Zijlstra <peterz@infradead.org> 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!
Сontinuing 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
| |