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Date: Wed, 7 Nov 2012 11:11:24 +0800 From: Ming Lei <ming.lei@...onical.com> To: Andrew Morton <akpm@...ux-foundation.org> Cc: linux-kernel@...r.kernel.org, Alan Stern <stern@...land.harvard.edu>, Oliver Neukum <oneukum@...e.de>, Minchan Kim <minchan@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rjw@...k.pl>, Jens Axboe <axboe@...nel.dk>, "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org, linux-usb@...r.kernel.org, linux-pm@...r.kernel.org, linux-mm@...ck.org, Jiri Kosina <jiri.kosina@...e.com>, Mel Gorman <mel@....ul.ie>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>, Michal Hocko <mhocko@...e.cz>, Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH v4 1/6] mm: teach mm by current context info to not do I/O during memory allocation On Wed, Nov 7, 2012 at 7:23 AM, Andrew Morton <akpm@...ux-foundation.org> wrote: > > It's unclear from the description why we're also clearing __GFP_FS in > this situation. > > If we can avoid doing this then there will be a very small gain: there > are some situations in which a filesystem can clean pagecache without > performing I/O. Firstly, the patch follows the policy in the system suspend/resume situation, in which the __GFP_FS is cleared, and basically the problem is very similar with that in system PM path. Secondly, inside shrink_page_list(), pageout() may be triggered on dirty anon page if __GFP_FS is set. IMO, if performing I/O can be completely avoided when __GFP_FS is set, the flag can be kept, otherwise it is better to clear it in the situation. > > It doesn't appear that the patch will add overhead to the alloc/free > hotpaths, which is good. Thanks for previous Minchan's comment. > >> >> ... >> >> --- a/include/linux/sched.h >> +++ b/include/linux/sched.h >> @@ -1805,6 +1805,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t * >> #define PF_FROZEN 0x00010000 /* frozen for system suspend */ >> #define PF_FSTRANS 0x00020000 /* inside a filesystem transaction */ >> #define PF_KSWAPD 0x00040000 /* I am kswapd */ >> +#define PF_MEMALLOC_NOIO 0x00080000 /* Allocating memory without IO involved */ >> #define PF_LESS_THROTTLE 0x00100000 /* Throttle me less: I clean memory */ >> #define PF_KTHREAD 0x00200000 /* I am a kernel thread */ >> #define PF_RANDOMIZE 0x00400000 /* randomize virtual address space */ >> @@ -1842,6 +1843,15 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t * >> #define tsk_used_math(p) ((p)->flags & PF_USED_MATH) >> #define used_math() tsk_used_math(current) >> >> +#define memalloc_noio() (current->flags & PF_MEMALLOC_NOIO) >> +#define memalloc_noio_save(flag) do { \ >> + (flag) = current->flags & PF_MEMALLOC_NOIO; \ >> + current->flags |= PF_MEMALLOC_NOIO; \ >> +} while (0) >> +#define memalloc_noio_restore(flag) do { \ >> + current->flags = (current->flags & ~PF_MEMALLOC_NOIO) | flag; \ >> +} while (0) >> + > > Again with the ghastly macros. Please, do this properly in regular old > C, as previously discussed. It really doesn't matter what daft things > local_irq_save() did 20 years ago. Just do it right! OK, I will take inline function in -v5. > > Also, you can probably put the unlikely() inside memalloc_noio() and > avoid repeating it at all the callsites. > > And it might be neater to do: > > /* > * Nice comment goes here > */ > static inline gfp_t memalloc_noio_flags(gfp_t flags) > { > if (unlikely(current->flags & PF_MEMALLOC_NOIO)) > flags &= ~GFP_IOFS; > return flags; > } But without the check in callsites, some local variables will be write two times, so it is better to not do it. > >> * task->jobctl flags >> */ >> >> ... >> >> @@ -2304,6 +2304,12 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order, >> .gfp_mask = sc.gfp_mask, >> }; >> >> + if (unlikely(memalloc_noio())) { >> + gfp_mask &= ~GFP_IOFS; >> + sc.gfp_mask = gfp_mask; >> + shrink.gfp_mask = sc.gfp_mask; >> + } > > We can avoid writing to shrink.gfp_mask twice. And maybe sc.gfp_mask > as well. Unclear, I didn't think about it too hard ;) Yes, we can do it by initializing 'shrink' local variable just after the branch, so one writing is enough. Will do it in -v5. Thanks, -- Ming Lei -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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