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

[Peter Zijlstra <peterz@infradead.org>]

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:
> 
> 
>