[Devel] [PATCH VZ10 01/11] drivers/md/dm-rq: fix time accounting for request-based dm drivers

[Andrey Zhadchenko]

On 7/17/26 16:50, Pavel Tikhomirov wrote:
> 
> 
> On 7/13/26 02:36, Andrey Zhadchenko wrote:
>> Currently all requests for request-based dm drivers are falling in
>> the last bin for both normal and precise counting due to incorrect
>> calculations.
>>
>> Ordinary counting expects that start_time passed to dm_stats_account_io()
>> is an absolute value and substracts it from jiffies to get request time.
>> However rq_end_stats() calculates and passes request duration. So we
>> actually get jiffies - <request_duration_in_jiffies>, which is incorrect.
>> rq_end_stats() should not do the substraction.
>>
>> For precise calulation stats->aux_duration_ns is used. It is calculated
>> in dm_stats_account_io() like that:
>> stats_aux->duration_ns = ktime_to_ns(ktime_get()) - stats_aux->duration_ns;
>> The problem is that stats_aux->duration_ns() is set by
>> dm_stats_record_start() which is called only for bio-based targets.
>> It should also be called in dm_start_request().
>>
>> https://virtuozzo.atlassian.net/browse/VSTOR-103846
>> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
>> ---
>>   drivers/md/dm-rq.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
>> index 2177666c64f32..58ad4c94d6119 100644
>> --- a/drivers/md/dm-rq.c
>> +++ b/drivers/md/dm-rq.c
>> @@ -129,7 +129,6 @@ static void rq_end_stats(struct mapped_device *md, struct request *orig)
>>   	if (unlikely(dm_stats_used(&md->stats))) {
>>   		struct dm_rq_target_io *tio = tio_from_request(orig);
>>   
>> -		tio->duration_jiffies = jiffies - tio->duration_jiffies;
> 
> Having variable called duration but having start time in it is a mess, can we rename?

Well I also don't like that, but it's mainstream design.

> 
> Plus this seems like mainstream bug.

It is. I will try sending the whole patchset (minus total_hist) a bit later.

> 
> Also in dm_start_request() probably the same problem, there jiffies-0 will be "duration".
> 
>>   		dm_stats_account_io(&md->stats, rq_data_dir(orig),
>>   				    blk_rq_pos(orig), tio->n_sectors, true,
>>   				    tio->duration_jiffies, &tio->stats_aux);
>> @@ -448,6 +447,8 @@ static void dm_start_request(struct mapped_device *md, struct request *orig)
>>   
>>   		tio->duration_jiffies = jiffies;
>>   		tio->n_sectors = blk_rq_sectors(orig);
>> +
>> +		dm_stats_record_start(&md->stats, &tio->stats_aux);
> 
> 
> 
>>   		dm_stats_account_io(&md->stats, rq_data_dir(orig),
>>   				    blk_rq_pos(orig), tio->n_sectors, false, 0,
>>   				    &tio->stats_aux);
>