Re: [PATCH] blk-throttle: track read and write request individually

[Shaohua Li <shli@kernel.org>]

On Wed, Oct 11, 2017 at 05:53:34PM +0800, Joseph Qi wrote:
> From: Joseph Qi <qijiang.qj@alibaba-inc.com>
> 
> In mixed read/write workload on SSD, write latency is much lower than
> read. But now we only track and record read latency and then use it as
> threshold base for both read and write io latency accounting. As a
> result, write io latency will always be considered as good and
> bad_bio_cnt is much smaller than 20% of bio_cnt. That is to mean, the
> tg to be checked will be treated as idle most of the time and still let
> others dispatch more ios, even it is truly running under low limit and
> wants its low limit to be guaranteed, which is not we expected in fact.
> So track read and write request individually, which can bring more
> precise latency control for low limit idle detection.

Looks good. Originally I thought we don't need very precise latency control, so
the read/write base latency difference doesn't matter too much, but tracking
write base latency shouldn't harm.
 
> Signed-off-by: Joseph Qi <qijiang.qj@alibaba-inc.com>
> ---
>  block/blk-throttle.c | 134 ++++++++++++++++++++++++++++++---------------------
>  1 file changed, 79 insertions(+), 55 deletions(-)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 17816a0..0897378 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -215,9 +215,9 @@ struct throtl_data
>  
>  	unsigned int scale;
>  
> -	struct latency_bucket tmp_buckets[LATENCY_BUCKET_SIZE];
> -	struct avg_latency_bucket avg_buckets[LATENCY_BUCKET_SIZE];
> -	struct latency_bucket __percpu *latency_buckets;
> +	struct latency_bucket tmp_buckets[2][LATENCY_BUCKET_SIZE];
> +	struct avg_latency_bucket avg_buckets[2][LATENCY_BUCKET_SIZE];
> +	struct latency_bucket __percpu *latency_buckets[2];
>  	unsigned long last_calculate_time;
>  	unsigned long filtered_latency;
>  
> @@ -2040,10 +2040,10 @@ static void blk_throtl_update_idletime(struct throtl_grp *tg)
>  #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
>  static void throtl_update_latency_buckets(struct throtl_data *td)
>  {
> -	struct avg_latency_bucket avg_latency[LATENCY_BUCKET_SIZE];
> -	int i, cpu;
> -	unsigned long last_latency = 0;
> -	unsigned long latency;
> +	struct avg_latency_bucket avg_latency[2][LATENCY_BUCKET_SIZE];
> +	int i, cpu, rw;
> +	unsigned long last_latency[2] = { 0 };
> +	unsigned long latency[2];
>  
>  	if (!blk_queue_nonrot(td->queue))
>  		return;
> @@ -2052,56 +2052,67 @@ static void throtl_update_latency_buckets(struct throtl_data *td)
>  	td->last_calculate_time = jiffies;
>  
>  	memset(avg_latency, 0, sizeof(avg_latency));
> -	for (i = 0; i < LATENCY_BUCKET_SIZE; i++) {
> -		struct latency_bucket *tmp = &td->tmp_buckets[i];
> -
> -		for_each_possible_cpu(cpu) {
> -			struct latency_bucket *bucket;
> -
> -			/* this isn't race free, but ok in practice */
> -			bucket = per_cpu_ptr(td->latency_buckets, cpu);
> -			tmp->total_latency += bucket[i].total_latency;
> -			tmp->samples += bucket[i].samples;
> -			bucket[i].total_latency = 0;
> -			bucket[i].samples = 0;
> -		}
> +	for (rw = READ; rw <= WRITE; rw++) {
> +		for (i = 0; i < LATENCY_BUCKET_SIZE; i++) {
> +			struct latency_bucket *tmp = &td->tmp_buckets[rw][i];
> +
> +			for_each_possible_cpu(cpu) {
> +				struct latency_bucket *bucket;
> +
> +				/* this isn't race free, but ok in practice */
> +				bucket = per_cpu_ptr(td->latency_buckets[rw],
> +					cpu);
> +				tmp->total_latency += bucket[i].total_latency;
> +				tmp->samples += bucket[i].samples;
> +				bucket[i].total_latency = 0;
> +				bucket[i].samples = 0;
> +			}
>  
> -		if (tmp->samples >= 32) {
> -			int samples = tmp->samples;
> +			if (tmp->samples >= 32) {
> +				int samples = tmp->samples;
>  
> -			latency = tmp->total_latency;
> +				latency[rw] = tmp->total_latency;
>  
> -			tmp->total_latency = 0;
> -			tmp->samples = 0;
> -			latency /= samples;
> -			if (latency == 0)
> -				continue;
> -			avg_latency[i].latency = latency;
> +				tmp->total_latency = 0;
> +				tmp->samples = 0;
> +				latency[rw] /= samples;
> +				if (latency[rw] == 0)
> +					continue;
> +				avg_latency[rw][i].latency = latency[rw];
> +			}
>  		}
>  	}
>  
> -	for (i = 0; i < LATENCY_BUCKET_SIZE; i++) {
> -		if (!avg_latency[i].latency) {
> -			if (td->avg_buckets[i].latency < last_latency)
> -				td->avg_buckets[i].latency = last_latency;
> -			continue;
> -		}
> +	for (rw = READ; rw <= WRITE; rw++) {
> +		for (i = 0; i < LATENCY_BUCKET_SIZE; i++) {
> +			if (!avg_latency[rw][i].latency) {
> +				if (td->avg_buckets[rw][i].latency < last_latency[rw])
> +					td->avg_buckets[rw][i].latency =
> +						last_latency[rw];
> +				continue;
> +			}
>  
> -		if (!td->avg_buckets[i].valid)
> -			latency = avg_latency[i].latency;
> -		else
> -			latency = (td->avg_buckets[i].latency * 7 +
> -				avg_latency[i].latency) >> 3;
> +			if (!td->avg_buckets[rw][i].valid)
> +				latency[rw] = avg_latency[rw][i].latency;
> +			else
> +				latency[rw] = (td->avg_buckets[rw][i].latency * 7 +
> +					avg_latency[rw][i].latency) >> 3;
>  
> -		td->avg_buckets[i].latency = max(latency, last_latency);
> -		td->avg_buckets[i].valid = true;
> -		last_latency = td->avg_buckets[i].latency;
> +			td->avg_buckets[rw][i].latency = max(latency[rw],
> +				last_latency[rw]);
> +			td->avg_buckets[rw][i].valid = true;
> +			last_latency[rw] = td->avg_buckets[rw][i].latency;
> +		}
>  	}
>  
>  	for (i = 0; i < LATENCY_BUCKET_SIZE; i++)
>  		throtl_log(&td->service_queue,
> -			"Latency bucket %d: latency=%ld, valid=%d", i,
> -			td->avg_buckets[i].latency, td->avg_buckets[i].valid);
> +			"Latency bucket %d: read latency=%ld, read valid=%d, "
> +			"write latency=%ld, write valid=%d", i,
> +			td->avg_buckets[READ][i].latency,
> +			td->avg_buckets[READ][i].valid,
> +			td->avg_buckets[WRITE][i].latency,
> +			td->avg_buckets[WRITE][i].valid);
>  }
>  #else
>  static inline void throtl_update_latency_buckets(struct throtl_data *td)
> @@ -2244,16 +2255,17 @@ static void throtl_track_latency(struct throtl_data *td, sector_t size,
>  	struct latency_bucket *latency;
>  	int index;
>  
> -	if (!td || td->limit_index != LIMIT_LOW || op != REQ_OP_READ ||
> +	if (!td || td->limit_index != LIMIT_LOW ||
> +	    !(op & (REQ_OP_READ | REQ_OP_WRITE)) ||

probably check op == REQ_OP_READ || op == REQ_OP_WRITE, discard also includes
the REQ_OP_WRITE bit for example. It doesn't make sense to include discard here.


Thanks,
Shaohua