On Fri, Jan 23, 2015 at 06:53:32PM +0300, Kirill Tkhai wrote: > It's useless to send reschedule interrupts in such situations. The earliest > point, where schedule() call is possible, is sysret_careful(). But in that > function we directly test TIF_NEED_RESCHED. > > So it's possible to get rid of that type of interrupts. > > How about this idea? Is set_bit() cheap on x86 machines?
So you set TIF_POLLING_NRFLAG on syscall entry and clear it again on exit? Thereby we avoid the IPI, because the exit path already checks for TIF_NEED_RESCHED. Should work I suppose, but I'm not too familiar with all that entry.S muck. Andy might know and appreciate this. > --- > 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 > 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 > movl $_TIF_ALLWORK_MASK,%edi > /* edi: flagmask */ > sysret_check: > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
