Re: [bug-report] possible performance problem in ret_to_user_from_irq

[Hui Tang]

On 2023/1/3 22:59, Jens Axboe wrote:
> On 1/3/23 7:34?AM, Russell King (Oracle) wrote:
> > 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.
>
> Hui Tang, can you give it a whirl? Just checked and it applies to
> 5.10-stable as well, just with a slight offset.

Okay, I'll test it today.