Re: [PATCH 1/3] block: delete part_round_stats and switch to less precise counting

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Nov 30 2018 at  2:42pm -0500,
Mike Snitzer <snitzer@redhat.com> wrote:

> On Tue, Nov 27 2018 at  7:42pm -0500,
> Mikulas Patocka <mpatocka@redhat.com> wrote:
> 
> > We want to convert to per-cpu in_flight counters.
> > 
> > The function part_round_stats needs the in_flight counter every jiffy, it
> > would be too costly to sum all the percpu variables every jiffy, so it
> > must be deleted. part_round_stats is used to calculate two counters -
> > time_in_queue and io_ticks.
> > 
> > time_in_queue can be calculated without part_round_stats, by adding the
> > duration of the I/O when the I/O ends (the value is almost as exact as the
> > previously calculated value, except that time for in-progress I/Os is not
> > counted).
> > 
> > io_ticks can be approximated by increasing the value when I/O is started
> > or ended and the jiffies value has changed. If the I/Os take less than a
> > jiffy, the value is as exact as the previously calculated value. If the
> > I/Os take more than a jiffy, io_ticks can drift behind the previously
> > calculated value.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > 
> > ---
> >  block/bio.c               |   26 ++++++++++++++++--
> >  block/blk-core.c          |   64 +++-------------------------------------------
> >  block/blk-merge.c         |    1 
> >  block/genhd.c             |    4 --
> >  block/partition-generic.c |    4 --
> >  include/linux/genhd.h     |    3 --
> >  6 files changed, 29 insertions(+), 73 deletions(-)
> > 
> > Index: linux-block/block/bio.c
> > ===================================================================
> > --- linux-block.orig/block/bio.c	2018-11-26 23:44:17.000000000 +0100
> > +++ linux-block/block/bio.c	2018-11-26 23:44:17.000000000 +0100
> > @@ -1663,13 +1663,29 @@ defer:
> >  }
> >  EXPORT_SYMBOL_GPL(bio_check_pages_dirty);
> >  
> > +void update_io_ticks(int cpu, struct hd_struct *part, unsigned long now)
> > +{
> > +	unsigned long stamp;
> > +again:
> > +	stamp = READ_ONCE(part->stamp);
> > +	if (unlikely(stamp != now)) {
> > +		if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) {
> > +			__part_stat_add(cpu, part, io_ticks, 1);
> > +		}
> > +	}
> > +	if (part->partno) {
> > +		part = &part_to_disk(part)->part0;
> > +		goto again;
> > +	}
> > +}
> > +
> >  void generic_start_io_acct(struct request_queue *q, int op,
> >  			   unsigned long sectors, struct hd_struct *part)
> >  {
> >  	const int sgrp = op_stat_group(op);
> >  	int cpu = part_stat_lock();
> >  
> > -	part_round_stats(q, cpu, part);
> > +	update_io_ticks(cpu, part, jiffies);
> >  	part_stat_inc(cpu, part, ios[sgrp]);
> >  	part_stat_add(cpu, part, sectors[sgrp], sectors);
> >  	part_inc_in_flight(q, part, op_is_write(op));
> > @@ -1681,12 +1697,16 @@ EXPORT_SYMBOL(generic_start_io_acct);
> >  void generic_end_io_acct(struct request_queue *q, int req_op,
> >  			 struct hd_struct *part, unsigned long start_time)
> >  {
> > -	unsigned long duration = jiffies - start_time;
> > +	unsigned long now = jiffies;
> > +	unsigned long duration = now - start_time;
> >  	const int sgrp = op_stat_group(req_op);
> >  	int cpu = part_stat_lock();
> >  
> > +	update_io_ticks(cpu, part, now);
> >  	part_stat_add(cpu, part, nsecs[sgrp], jiffies_to_nsecs(duration));
> > -	part_round_stats(q, cpu, part);
> > +	part_stat_add(cpu, part, time_in_queue, duration);
> > +	if (part->partno)
> > +		part_stat_add(cpu, &part_to_disk(part)->part0, time_in_queue, duration);
> >  	part_dec_in_flight(q, part, op_is_write(req_op));
> >  
> >  	part_stat_unlock();
> > Index: linux-block/block/blk-core.c
> > ===================================================================
> > --- linux-block.orig/block/blk-core.c	2018-11-26 23:44:17.000000000 +0100
> > +++ linux-block/block/blk-core.c	2018-11-26 23:44:17.000000000 +0100
> > @@ -583,63 +583,6 @@ struct request *blk_get_request(struct r
> >  }
> >  EXPORT_SYMBOL(blk_get_request);
> >  
> > -static void part_round_stats_single(struct request_queue *q, int cpu,
> > -				    struct hd_struct *part, unsigned long now,
> > -				    unsigned int inflight)
> > -{
> > -	if (inflight) {
> > -		__part_stat_add(cpu, part, time_in_queue,
> > -				inflight * (now - part->stamp));
> > -		__part_stat_add(cpu, part, io_ticks, (now - part->stamp));
> > -	}
> > -	part->stamp = now;
> > -}
> > -
> > -/**
> > - * part_round_stats() - Round off the performance stats on a struct disk_stats.
> > - * @q: target block queue
> > - * @cpu: cpu number for stats access
> > - * @part: target partition
> > - *
> > - * The average IO queue length and utilisation statistics are maintained
> > - * by observing the current state of the queue length and the amount of
> > - * time it has been in this state for.
> > - *
> > - * Normally, that accounting is done on IO completion, but that can result
> > - * in more than a second's worth of IO being accounted for within any one
> > - * second, leading to >100% utilisation.  To deal with that, we call this
> > - * function to do a round-off before returning the results when reading
> > - * /proc/diskstats.  This accounts immediately for all queue usage up to
> > - * the current jiffies and restarts the counters again.
> > - */
> > -void part_round_stats(struct request_queue *q, int cpu, struct hd_struct *part)
> > -{
> > -	struct hd_struct *part2 = NULL;
> > -	unsigned long now = jiffies;
> > -	unsigned int inflight[2];
> > -	int stats = 0;
> > -
> > -	if (part->stamp != now)
> > -		stats |= 1;
> > -
> > -	if (part->partno) {
> > -		part2 = &part_to_disk(part)->part0;
> > -		if (part2->stamp != now)
> > -			stats |= 2;
> > -	}
> > -
> > -	if (!stats)
> > -		return;
> > -
> > -	part_in_flight(q, part, inflight);
> > -
> > -	if (stats & 2)
> > -		part_round_stats_single(q, cpu, part2, now, inflight[1]);
> > -	if (stats & 1)
> > -		part_round_stats_single(q, cpu, part, now, inflight[0]);
> > -}
> > -EXPORT_SYMBOL_GPL(part_round_stats);
> > -
> >  void blk_put_request(struct request *req)
> >  {
> >  	blk_mq_free_request(req);
> > @@ -1408,9 +1351,11 @@ void blk_account_io_done(struct request
> >  		cpu = part_stat_lock();
> >  		part = req->part;
> >  
> > +		update_io_ticks(cpu, part, jiffies);
> >  		part_stat_inc(cpu, part, ios[sgrp]);
> >  		part_stat_add(cpu, part, nsecs[sgrp], now - req->start_time_ns);
> > -		part_round_stats(req->q, cpu, part);
> > +		part_stat_add(cpu, part, time_in_queue, nsecs_to_jiffies64(now - req->start_time_ns));
> 
> (Extra indentation offered a clue...)
> Are you missing an 'if (part->partno)' conditional here?
> 
> > +			part_stat_add(cpu, &part_to_disk(part)->part0, time_in_queue, nsecs_to_jiffies64(now - req->start_time_ns));
> >  		part_dec_in_flight(req->q, part, rq_data_dir(req));
> >  
> >  		hd_struct_put(part);
> 

part_stat_add() already deals with the 'if (part->partno)' case.  So you
were double accounting -- here and in generic_end_io_acct()