[dm-devel] [PATCH 2/3] block: switch to per-cpu in-flight counters

[Jens Axboe]

On 11/30/18 2:44 PM, Mike Snitzer wrote:
> On Tue, Nov 27 2018 at  7:42pm -0500,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
> 
>> Now when part_round_stats is gone, we can switch to per-cpu in-flight
>> counters.
>>
>> We use the local-atomic type local_t, so that if part_inc_in_flight or
>> part_dec_in_flight is reentrantly called from an interrupt, the value will
>> be correct.
>>
>> The other counters could be corrupted due to reentrant interrupt, but the
>> corruption only results in slight counter skew - the in_flight counter
>> must be exact, so it needs local_t.
>>
>> Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
>>
>> ---
>>  block/bio.c           |    4 ++--
>>  block/blk-core.c      |    4 ++--
>>  block/blk-merge.c     |    2 +-
>>  block/genhd.c         |   47 +++++++++++++++++++++++++++++++++++------------
>>  drivers/md/dm.c       |    4 +---
>>  include/linux/genhd.h |    7 ++++---
>>  6 files changed, 45 insertions(+), 23 deletions(-)
>>
>> Index: linux-block/block/genhd.c
>> ===================================================================
>> --- linux-block.orig/block/genhd.c	2018-11-28 00:09:59.000000000 +0100
>> +++ linux-block/block/genhd.c	2018-11-28 00:09:59.000000000 +0100
>> @@ -45,53 +45,76 @@ static void disk_add_events(struct gendi
>>  static void disk_del_events(struct gendisk *disk);
>>  static void disk_release_events(struct gendisk *disk);
>>  
>> -void part_inc_in_flight(struct request_queue *q, struct hd_struct *part, int rw)
>> +void part_inc_in_flight(struct request_queue *q, int cpu, struct hd_struct *part, int rw)
>>  {
>>  	if (queue_is_mq(q))
>>  		return;
>>  
>> -	atomic_inc(&part->in_flight[rw]);
>> +	local_inc(per_cpu_ptr(part->dkstats, cpu)->in_flight);
>>  	if (part->partno)
>> -		atomic_inc(&part_to_disk(part)->part0.in_flight[rw]);
>> +		local_inc(per_cpu_ptr(part_to_disk(part)->part0.dkstats, cpu)->in_flight);
>>  }
>>  
>> -void part_dec_in_flight(struct request_queue *q, struct hd_struct *part, int rw)
>> +void part_dec_in_flight(struct request_queue *q, int cpu, struct hd_struct *part, int rw)
>>  {
>>  	if (queue_is_mq(q))
>>  		return;
>>  
>> -	atomic_dec(&part->in_flight[rw]);
>> +	local_dec(per_cpu_ptr(part->dkstats, cpu)->in_flight);
>>  	if (part->partno)
>> -		atomic_dec(&part_to_disk(part)->part0.in_flight[rw]);
>> +		local_dec(per_cpu_ptr(part_to_disk(part)->part0.dkstats, cpu)->in_flight);
>>  }
> 
> 
> The above is wrong, needs this incremental fix:

Should also be using this_cpu_inc/dec instead.

-- 
Jens Axboe