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? > >> >> >> 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. > >> 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. --Andy -- 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/
