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

[Josef Bacik <josef@toxicpanda.com>]

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