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Message-ID: <34fbafbe-a510-5193-b86c-91fac69de95f@huaweicloud.com>
Date: Mon, 26 Feb 2024 17:12:44 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Xiao Ni <xni@...hat.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc: paul.e.luse@...ux.intel.com, song@...nel.org, neilb@...e.com,
shli@...com, linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
yi.zhang@...wei.com, yangerkun@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH md-6.9 03/10] md/raid1: fix choose next idle in
read_balance()
Hi,
在 2024/02/26 16:55, Xiao Ni 写道:
> Hi Kuai
>
> Thanks for the effort!
>
> On Thu, Feb 22, 2024 at 4:04 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>>
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> Commit 12cee5a8a29e ("md/raid1: prevent merging too large request") add
>> the case choose next idle in read_balance():
>>
>> read_balance:
>> for_each_rdev
>> if(next_seq_sect == this_sector || disk == 0)
>
> typo error: s/disk/dist/g
>
>> -> sequential reads
>> best_disk = disk;
>> if (...)
>> choose_next_idle = 1
>> continue;
>>
>> for_each_rdev
>> -> iterate next rdev
>> if (pending == 0)
>> best_disk = disk;
>> -> choose the next idle disk
>> break;
>>
>> if (choose_next_idle)
>> -> keep using this rdev if there are no other idle disk
>> continue
>>
>> However, commit 2e52d449bcec ("md/raid1: add failfast handling for reads.")
>> remove the code:
>>
>> - /* If device is idle, use it */
>> - if (pending == 0) {
>> - best_disk = disk;
>> - break;
>> - }
>>
>> Hence choose next idle will never work now, fix this problem by
>> following:
>>
>> 1) don't set best_disk in this case, read_balance() will choose the best
>> disk after iterating all the disks;
>> 2) add 'pending' so that other idle disk will be chosen;
>> 3) set 'dist' to 0 so that if there is no other idle disk, and all disks
>> are rotational, this disk will still be chosen;
>>
>> Fixes: 2e52d449bcec ("md/raid1: add failfast handling for reads.")
>> Co-developed-by: Paul Luse <paul.e.luse@...ux.intel.com>
>> Signed-off-by: Paul Luse <paul.e.luse@...ux.intel.com>
>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>> ---
>> drivers/md/raid1.c | 21 ++++++++++++---------
>> 1 file changed, 12 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
>> index c60ea58ae8c5..d0bc67e6d068 100644
>> --- a/drivers/md/raid1.c
>> +++ b/drivers/md/raid1.c
>> @@ -604,7 +604,6 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
>> unsigned int min_pending;
>> struct md_rdev *rdev;
>> int choose_first;
>> - int choose_next_idle;
>>
>> /*
>> * Check if we can balance. We can balance on the whole
>> @@ -619,7 +618,6 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
>> best_pending_disk = -1;
>> min_pending = UINT_MAX;
>> best_good_sectors = 0;
>> - choose_next_idle = 0;
>> clear_bit(R1BIO_FailFast, &r1_bio->state);
>>
>> if ((conf->mddev->recovery_cp < this_sector + sectors) ||
>> @@ -712,7 +710,6 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
>> int opt_iosize = bdev_io_opt(rdev->bdev) >> 9;
>> struct raid1_info *mirror = &conf->mirrors[disk];
>>
>> - best_disk = disk;
>> /*
>> * If buffered sequential IO size exceeds optimal
>> * iosize, check if there is idle disk. If yes, choose
>> @@ -731,15 +728,21 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
>> mirror->next_seq_sect > opt_iosize &&
>> mirror->next_seq_sect - opt_iosize >=
>> mirror->seq_start) {
>> - choose_next_idle = 1;
>> - continue;
>> + /*
>> + * Add 'pending' to avoid choosing this disk if
>> + * there is other idle disk.
>> + * Set 'dist' to 0, so that if there is no other
>> + * idle disk and all disks are rotational, this
>> + * disk will still be chosen.
>> + */
>> + pending++;
>> + dist = 0;
>
> There is a problem. If all disks are not idle and there is a disk with
> dist=0 before the seq disk, it can't read from the seq disk. It will
> read from the first disk with dist=0. Maybe we can only add the codes
> which are removed from 2e52d449bcec?
If there is a disk with disk=0, then best_dist_disk will be updated to
the disk, and best_dist will be updated to 0 already:
// in each iteration
if (dist < best_dist) {
best_dist = dist;
btest_disk_disk = disk;
}
In this case, best_dist will be set to the first disk with dist=0, and
at last, the disk will be chosen:
if (best_disk == -1) {
if (has_nonrot_disk || min_pending == 0)
best_disk = best_pending_disk;
else
best_disk = best_dist_disk;
-> the first disk with dist=0;
}
So, the problem that you concerned should not exist.
Thanks,
Kuai
>
> Best Regards
> Xiao
>
>> + } else {
>> + best_disk = disk;
>> + break;
>> }
>> - break;
>> }
>>
>> - if (choose_next_idle)
>> - continue;
>> -
>> if (min_pending > pending) {
>> min_pending = pending;
>> best_pending_disk = disk;
>> --
>> 2.39.2
>>
>>
>
> .
>
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