linux-kernel - Re: [PATCH 08/24] mm/swap: check readahead policy per entry

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Date:   Thu, 23 Nov 2023 18:52:06 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     Chris Li <chrisl@...nel.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ying" <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/24] mm/swap: check readahead policy per entry

Chris Li <chrisl@...nel.org> 于2023年11月21日周二 15:54写道：
>
> On Sun, Nov 19, 2023 at 11:48 AM Kairui Song <ryncsn@...il.com> wrote:
> >
> > From: Kairui Song <kasong@...cent.com>
> >
> > Currently VMA readahead is globally disabled when any rotate disk is
> > used as swap backend. So multiple swap devices are enabled, if a slower
> > hard disk is set as a low priority fallback, and a high performance SSD
> > is used and high priority swap device, vma readahead is disabled globally.
> > The SSD swap device performance will drop by a lot.
> >
> > Check readahead policy per entry to avoid such problem.
> >
> > Signed-off-by: Kairui Song <kasong@...cent.com>
> > ---
> >  mm/swap_state.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index ff6756f2e8e4..fb78f7f18ed7 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -321,9 +321,9 @@ static inline bool swap_use_no_readahead(struct swap_info_struct *si, swp_entry_
> >         return data_race(si->flags & SWP_SYNCHRONOUS_IO) && __swap_count(entry) == 1;
> >  }
> >
> > -static inline bool swap_use_vma_readahead(void)
> > +static inline bool swap_use_vma_readahead(struct swap_info_struct *si)
> >  {
> > -       return READ_ONCE(enable_vma_readahead) && !atomic_read(&nr_rotate_swap);
> > +       return data_race(si->flags & SWP_SOLIDSTATE) && READ_ONCE(enable_vma_readahead);
>
> A very minor point:
> I notice you change the order enable_vma_readahead to the last.
> Normally if enable_vma_reachahead == 0, there is no need to check the si->flags.
> The si->flags check is more expensive than simple memory load.
> You might want to check enable_vma_readahead first then you can short
> cut the more expensive part.

Thanks, I'll improve this part.