Re: [PATCH] mm: annotate refault stalls from swap_readpage

[Shakeel Butt <shakeelb@google.com>]

On Thu, Oct 10, 2019 at 3:11 PM Minchan Kim <minchan@kernel.org> wrote:
>
> On Thu, Oct 10, 2019 at 03:17:47PM -0400, Johannes Weiner wrote:
> > On Thu, Oct 10, 2019 at 08:21:34AM -0700, Minchan Kim wrote:
> > > From: Minchan Kim <minchan@google.com>
> > >
> > > If block device supports rw_page operation, it doesn't submit bio
> > > so annotation in submit_bio for refault stall doesn't work.
> > > It happens with zram in android, especially swap read path which
> > > could consume CPU cycle for decompress. It is also a problem for
> > > zswap which uses frontswap.
> > >
> > > Annotate swap_readpage() to account the synchronous IO overhead
> > > to prevent underreport memory pressure.
> > >
> > > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > > Signed-off-by: Minchan Kim <minchan@google.com>
> >
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>
> Thanks, Johannes!
>
> >
> > Can you please add a comment to the caller? Lifted from submit_bio():
>
> Sure, I added a little about zram.
>
> >
> >       /*
> >        * Count submission time as memory stall. When the device is
> >        * congested, or the submitting cgroup IO-throttled,
> >        * submission can be a significant part of overall IO time.
> >        */
>
>
> From a8ae7cbc2d3f050aca810fd68285d45cb933b825 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@google.com>
> Date: Thu, 10 Oct 2019 08:09:06 -0700
> Subject: [PATCH v2] mm: annotate refault stalls from swap_readpage
>
> If block device supports rw_page operation, it doesn't submit bio
> so annotation in submit_bio for refault stall doesn't work.
> It happens with zram in android, especially swap read path which
> could consume CPU cycle for decompress. It is also a problem for
> zswap which uses frontswap.
>
> Annotate swap_readpage() to account the synchronous IO overhead
> to prevent underreport memory pressure.
>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Minchan Kim <minchan@google.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>

> ---
>  mm/page_io.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 24ee600f9131..18f1f8e1d27f 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -22,6 +22,7 @@
>  #include <linux/writeback.h>
>  #include <linux/frontswap.h>
>  #include <linux/blkdev.h>
> +#include <linux/psi.h>
>  #include <linux/uio.h>
>  #include <linux/sched/task.h>
>  #include <asm/pgtable.h>
> @@ -354,10 +355,20 @@ int swap_readpage(struct page *page, bool synchronous)
>         struct swap_info_struct *sis = page_swap_info(page);
>         blk_qc_t qc;
>         struct gendisk *disk;
> +       unsigned long pflags;
>
>         VM_BUG_ON_PAGE(!PageSwapCache(page) && !synchronous, page);
>         VM_BUG_ON_PAGE(!PageLocked(page), page);
>         VM_BUG_ON_PAGE(PageUptodate(page), page);
> +
> +       /*
> +        * Count submission time as memory stall. When the device is
> +        * congested, the submitting cgroup IO-throttled, or backing
> +        * device works synchronously(e.g., zram), submission can be
> +        * a significant part of overall IO time.
> +        */
> +       psi_memstall_enter(&pflags);
> +
>         if (frontswap_load(page) == 0) {
>                 SetPageUptodate(page);
>                 unlock_page(page);
> @@ -371,7 +382,7 @@ int swap_readpage(struct page *page, bool synchronous)
>                 ret = mapping->a_ops->readpage(swap_file, page);
>                 if (!ret)
>                         count_vm_event(PSWPIN);
> -               return ret;
> +               goto out;
>         }
>
>         ret = bdev_read_page(sis->bdev, swap_page_sector(page), page);
> @@ -382,7 +393,7 @@ int swap_readpage(struct page *page, bool synchronous)
>                 }
>
>                 count_vm_event(PSWPIN);
> -               return 0;
> +               goto out;
>         }
>
>         ret = 0;
> @@ -418,6 +429,7 @@ int swap_readpage(struct page *page, bool synchronous)
>         bio_put(bio);
>
>  out:
> +       psi_memstall_leave(&pflags);
>         return ret;
>  }
>
> --
> 2.23.0.581.g78d2f28ef7-goog
>