Re: [bug-report] possible performance problem in ret_to_user_from_irq
From: Russell King (Oracle)
Date: Tue Jan 03 2023 - 09:34:40 EST
On Tue, Jan 03, 2023 at 07:25:26AM -0700, Jens Axboe wrote:
> On 1/3/23 3:06?AM, Russell King (Oracle) wrote:
> > On Mon, Dec 26, 2022 at 04:45:20PM +0800, Hui Tang wrote:
> >> hi folks.
> >>
> >> I found a performance problem which is introduced by commit
> >> 32d59773da38 ("arm: add support for TIF_NOTIFY_SIGNAL").
> >> After the commit, any bit in the range of 0..15 will cause
> >> do_work_pending() to be invoked. More frequent do_work_pending()
> >> invoked possible result in worse performance.
> >>
> >> Some of the tests I've done? as follows:
> >> lmbench test base with patch
> >> ./lat_ctx -P 1 -s 0 2 7.3167 11.04
> >> ./lat_ctx -P 1 -s 16 2 8.0467 14.5367
> >> ./lat_ctx -P 1 -s 64 2 7.8667 11.43
> >> ./lat_ctx -P 1 -s 16 16 16.47 18.3667
> >> ./lat_pipe -P 1 28.1671 44.7904
> >>
> >> libMicro-0.4.1 test base with patch
> >> ./cascade_cond -E -C 200\
> >> -L -S -W -N "c_cond_1" -I 100 286.3333 358
> >>
> >> When I adjust test bit, the performance problem gone.
> >> - movs r1, r1, lsl #16
> >> + ldr r2, =#_TIF_WORK_MASK
> >> + tst r1, r2
> >>
> >> Does anyone have a good suggestion for this problem?
> >> should just test _TIF_WORK_MASK, as before?
> >
> > I think it should be fine - but I would suggest re-organising the
> > TIF definitions so that those TIF bits that shouldn't trigger
> > do_work_pending are not in the first 16 bits.
> >
> > Note that all four bits in _TIF_SYSCALL_WORK need to stay within
> > an 8-bit even-bit-aligned range, so the value is suitable for an
> > immediate assembly constant.
> >
> > I'd suggest moving the TIF definitions for 20 to 19, and 4..7 to
> > 20..23, and then 8 to 4.
>
> Like this?
>
> diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
> index aecc403b2880..7f092cb55a41 100644
> --- a/arch/arm/include/asm/thread_info.h
> +++ b/arch/arm/include/asm/thread_info.h
> @@ -128,15 +128,16 @@ extern int vfp_restore_user_hwstate(struct user_vfp *,
> #define TIF_NEED_RESCHED 1 /* rescheduling necessary */
> #define TIF_NOTIFY_RESUME 2 /* callback before returning to user */
> #define TIF_UPROBE 3 /* breakpointed or singlestepping */
> -#define TIF_SYSCALL_TRACE 4 /* syscall trace active */
> -#define TIF_SYSCALL_AUDIT 5 /* syscall auditing active */
> -#define TIF_SYSCALL_TRACEPOINT 6 /* syscall tracepoint instrumentation */
> -#define TIF_SECCOMP 7 /* seccomp syscall filtering active */
> -#define TIF_NOTIFY_SIGNAL 8 /* signal notifications exist */
> +#define TIF_NOTIFY_SIGNAL 4 /* signal notifications exist */
>
> #define TIF_USING_IWMMXT 17
> #define TIF_MEMDIE 18 /* is terminating due to OOM killer */
> -#define TIF_RESTORE_SIGMASK 20
> +#define TIF_RESTORE_SIGMASK 19
> +#define TIF_SYSCALL_TRACE 20 /* syscall trace active */
> +#define TIF_SYSCALL_AUDIT 21 /* syscall auditing active */
> +#define TIF_SYSCALL_TRACEPOINT 22 /* syscall tracepoint instrumentation */
> +#define TIF_SECCOMP 23 /* seccomp syscall filtering active */
> +
>
> #define _TIF_SIGPENDING (1 << TIF_SIGPENDING)
> #define _TIF_NEED_RESCHED (1 << TIF_NEED_RESCHED)
Yep, LGTM, thanks.
--
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