В Пт, 23/01/2015 в 18:36 -0800, Andy Lutomirski пишет: > On Fri, Jan 23, 2015 at 9:09 AM, Kirill Tkhai <ktk...@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 <pet...@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! Kirill -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
