Re: [PATCH v3 1/4] btrfs: add read_policy latency

Josef Bacik
2021-01-19T19:38:21Z

On 1/11/21 4:41 AM, Anand Jain wrote:
> The read policy type latency routes the read IO based on the historical
> average wait-time experienced by the read IOs through the individual
> device. This patch obtains the historical read IO stats from the kernel
> block layer and calculates its average.
>
> Example usage:
> echo "latency" > /sys/fs/btrfs/$uuid/read_policy
>
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
> v3: The block layer commit 0d02129e76ed (block: merge struct block_device and
> struct hd_struct) has changed the first argument in the function
> part_stat_read_all() in 5.11-rc1. So the compilation will fail. This patch
> fixes it.
> Commit log updated.
>
> v2: Use btrfs_debug_rl() instead of btrfs_info_rl()
> It is better we have this debug until we test this on at least few
> hardwares.
> Drop the unrelated changes.
> Update change log.
>
> v1: Drop part_stat_read_all instead use part_stat_read
> Drop inflight
>
>
> fs/btrfs/sysfs.c | 3 ++-
> fs/btrfs/volumes.c | 38 ++++++++++++++++++++++++++++++++++++++
> fs/btrfs/volumes.h | 2 ++
> 3 files changed, 42 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
> index 19b9fffa2c9c..96ca7bef6357 100644
> --- a/fs/btrfs/sysfs.c
> +++ b/fs/btrfs/sysfs.c
> @@ -915,7 +915,8 @@ static bool strmatch(const char *buffer, const char *string)
> return false;
> }
>
> -static const char * const btrfs_read_policy_name[] = { "pid" };
> +/* Must follow the order as in enum btrfs_read_policy */
> +static const char * const btrfs_read_policy_name[] = { "pid", "latency" };
>
> static ssize_t btrfs_read_policy_show(struct kobject *kobj,
> struct kobj_attribute *a, char *buf)
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index f4037a6bd926..f7a0a83d2cd4 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -14,6 +14,7 @@
> #include <linux/semaphore.h>
> #include <linux/uuid.h>
> #include <linux/list_sort.h>
> +#include <linux/part_stat.h>
> #include "misc.h"
> #include "ctree.h"
> #include "extent_map.h"
> @@ -5490,6 +5491,39 @@ int btrfs_is_parity_mirror(struct btrfs_fs_info *fs_info, u64 logical, u64 len)
> return ret;
> }
>
> +static int btrfs_find_best_stripe(struct btrfs_fs_info *fs_info,
> + struct map_lookup *map, int first,
> + int num_stripe)
> +{
> + u64 est_wait = 0;
> + int best_stripe = 0;
> + int index;
> +
> + for (index = first; index < first + num_stripe; index++) {
> + u64 read_wait;
> + u64 avg_wait = 0;
> + unsigned long read_ios;
> + struct btrfs_device *device = map->stripes[index].dev;
> +
> + read_wait = part_stat_read(device->bdev, nsecs[READ]);
> + read_ios = part_stat_read(device->bdev, ios[READ]);
> +
> + if (read_wait && read_ios && read_wait >= read_ios)
> + avg_wait = div_u64(read_wait, read_ios);
> + else
> + btrfs_debug_rl(device->fs_devices->fs_info,

You can just use fs_info here, you already have it. I'm not in love with doing
this check every time, I'd rather cache the results somewhere. However if we're
read-only I can't think of a mechanism we could piggy back on, RW we could just
do it every transaction commit. Fix the fs_info thing and you can add

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef