From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Alex Zhuravlev <azhuravlev@whamcloud.com>
Cc: "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>
Subject: Re: [PATCH 1/2] ext4: mballoc - prefetching for bitmaps
Date: Thu, 28 May 2020 10:47:46 -0400 [thread overview]
Message-ID: <20200528144746.GE228632@mit.edu> (raw)
In-Reply-To: <262A2973-9B2D-4DBE-8752-67E91D52C632@whamcloud.com>
On Fri, May 15, 2020 at 10:07:06AM +0000, Alex Zhuravlev wrote:
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2127,6 +2127,96 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
> return 0;
> }
>
> +/*
> + * each allocation context (i.e. a thread doing allocation) has own
> + * sliding prefetch window of @s_mb_prefetch size which starts at the
> + * very first goal and moves ahead of scaning.
> + * a side effect is that subsequent allocations will likely find
> + * the bitmaps in cache or at least in-flight.
> + */
> +static void
> +ext4_mb_prefetch(struct ext4_allocation_context *ac,
> + ext4_group_t start)
> +{
> + struct super_block *sb = ac->ac_sb;
> + ext4_group_t ngroups = ext4_get_groups_count(sb);
> + struct ext4_sb_info *sbi = EXT4_SB(sb);
> + struct ext4_group_info *grp;
> + ext4_group_t nr, group = start;
> + struct buffer_head *bh;
> +
> + /* limit prefetching at cr=0, otherwise mballoc can
> + * spend a lot of time loading imperfect groups */
> + if (ac->ac_criteria < 2 && ac->ac_prefetch_ios >= sbi->s_mb_prefetch_limit)
> + return;
> +
> + /* batch prefetching to get few READs in flight */
> + nr = ac->ac_prefetch - group;
> + if (ac->ac_prefetch < group)
> + /* wrapped to the first groups */
> + nr += ngroups;
> + if (nr > 0)I
> + return;
> + BUG_ON(nr < 0);
What are you trying to do here? If nr > 0, we return; if nr < 0, we
BUG() --- but nr is an unsigned int, so we never can trigger --- which
was the warning reported by the kbuild test bot. So we will only get
past this point if ac_prefetch == group. But ac_prefetch appears to
be the last group that we prefetched, so it's not clear that the logic
is correct here.
- Ted
next prev parent reply other threads:[~2020-05-28 14:47 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-15 10:07 [PATCH 1/2] ext4: mballoc - prefetching for bitmaps Alex Zhuravlev
2020-05-20 4:45 ` kbuild test robot
2020-05-23 19:27 ` kbuild test robot
2020-05-28 14:47 ` Theodore Y. Ts'o [this message]
2020-05-30 4:57 ` Alex Zhuravlev
2020-05-29 16:19 ` Благодаренко Артём
2020-05-30 5:01 ` Alex Zhuravlev
2020-06-19 17:16 ` Theodore Y. Ts'o
2020-06-19 18:23 ` Alex Zhuravlev
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