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Subject: Re: [PATCH md-6.9 03/10] md/raid1: fix choose next idle in
 read_balance()
To: Xiao Ni <xni@redhat.com>, Yu Kuai <yukuai1@huaweicloud.com>
Cc: paul.e.luse@linux.intel.com, song@kernel.org, neilb@suse.com,
 shli@fb.com, linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
 yi.zhang@huawei.com, yangerkun@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
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From: Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <05fab111-7111-1dca-57a8-89069a34adc2@huaweicloud.com>
Date: Mon, 26 Feb 2024 17:40:05 +0800
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Hi,

在 2024/02/26 17:24, Xiao Ni 写道:
> On Mon, Feb 26, 2024 at 5:12 PM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>>
>> 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@huaweicloud.com> wrote:
>>>>
>>>> From: Yu Kuai <yukuai3@huawei.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@linux.intel.com>
>>>> Signed-off-by: Paul Luse <paul.e.luse@linux.intel.com>
>>>> Signed-off-by: Yu Kuai <yukuai3@huawei.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.
> 
> Hi Kuai
> 
> Thanks for the explanation. You're right. It chooses the first disk
> which has dist==0. In the above, you made the same typo error disk=0
> as the comment. I guess you want to use dist=0, right? Beside this,
> this patch is good to me.

Yes, and Paul change the name 'best_dist' to 'closest_dist_disk',
and 'btest_disk_disk' to 'closest_dist' in the last patch to avoid typo
like this. :)

Thanks,
Kuai


> 
> Best Regards
> Xiao
>>
>> 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
>>>>
>>>>
>>>
>>> .
>>>
>>
> 
> .
>