linux-kernel - Re: [bug-report] possible performance problem in ret_to_user_from

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Message-ID: <Y7Q88aBpxfWRqzTe@shell.armlinux.org.uk>
Date:   Tue, 3 Jan 2023 14:34:25 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Hui Tang <tanghui20@...wei.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [bug-report] possible performance problem in ret_to_user_from_irq

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.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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