Re: 答复: [PATCH] Revert "block/mq-deadline: use correct way to throttling write requests"

[Bart Van Assche <bvanassche@acm.org>]

On 3/13/24 18:03, 牛志国 (Zhiguo Niu) wrote:
> Just as mentioned in original patch, "dd->async_depth = max(1UL, 3 * q->nr_requests / 4);", this limitation methods look likes won't have a limit effect, because tag allocated is based on sbitmap, not based the whole nr_requests.
> Right?
> Thanks!
> 
> For write requests, when we assign a tags from sched_tags,
> data->shallow_depth will be passed to sbitmap_find_bit,
> see the following code:
> 
> nr = sbitmap_find_bit_in_word(&sb->map[index],
> 			min_t (unsigned int,
> 			__map_depth(sb, index),
> 			depth),
> 			alloc_hint, wrap);
> 
> The smaller of data->shallow_depth and __map_depth(sb, index)
> will be used as the maximum range when allocating bits.
> 
> For a mmc device (one hw queue, deadline I/O scheduler):
> q->nr_requests = sched_tags = 128, so according to the previous
> calculation method, dd->async_depth = data->shallow_depth = 96,
> and the platform is 64bits with 8 cpus, sched_tags.bitmap_tags.sb.shift=5,
> sb.maps[]=32/32/32/32, 32 is smaller than 96, whether it is a read or
> a write I/O, tags can be allocated to the maximum range each time,
> which has not throttling effect.
Whether or not the code in my patch effectively performs throttling,
we need this revert to be merged. The patch that is being reverted
("block/mq-deadline: use correct way to throttling write requests")
ended up in Greg KH's stable branches. Hence, the first step is to
revert that patch and tag it with "Cc: stable" such that the revert
lands in the stable branches.

Thanks,

Bart.