Re: [PATCH v3 RESEND 2/2] blk-throttle: fix wrong initialization in case of dm device

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Jan 18 2018 at 10:09pm -0500,
Joseph Qi <joseph.qi@linux.alibaba.com> wrote:

> From: Joseph Qi <joseph.qi@linux.alibaba.com>
> 
> DM device sets QUEUE_FLAG_NONROT after the queue is registered. That is
> to mean, the previous initialization in blk_throtl_register_queue is
> wrong in this case.
> Fix it by checking and then updating the info during root tg
> initialization as we don't have a better choice.
> 
> Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
> Reviewed-by: Shaohua Li <shli@kernel.org>
> ---
>  block/blk-throttle.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index bf52035..7150f14 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -541,6 +541,25 @@ static void throtl_pd_init(struct blkg_policy_data *pd)
>  	if (cgroup_subsys_on_dfl(io_cgrp_subsys) && blkg->parent)
>  		sq->parent_sq = &blkg_to_tg(blkg->parent)->service_queue;
>  	tg->td = td;
> +
> +#ifdef CONFIG_BLK_DEV_THROTTLING_LOW
> +	/*
> +	 * DM device sets QUEUE_FLAG_NONROT after the queue is registered,
> +	 * so the previous initialization is wrong in this case. Check and
> +	 * update it here.
> +	 */
> +	if (blk_queue_nonrot(blkg->q) &&
> +	    td->filtered_latency != LATENCY_FILTERED_SSD) {
> +		int i;
> +
> +		td->throtl_slice = DFL_THROTL_SLICE_SSD;
> +		td->filtered_latency = LATENCY_FILTERED_SSD;
> +		for (i = 0; i < LATENCY_BUCKET_SIZE; i++) {
> +			td->avg_buckets[READ][i].latency = 0;
> +			td->avg_buckets[WRITE][i].latency = 0;
> +		}
> +	}
> +#endif
>  }
>  
>  /*
> -- 
> 1.9.4

This should be fixed for 4.16, please see these block tree commits:
http://git.kernel.dk/cgit/linux-block/commit/?h=for-4.16/block&id=fa70d2e2c4a0a54ced98260c6a176cc94c876d27
http://git.kernel.dk/cgit/linux-block/commit/?h=for-4.16/block&id=c100ec49fdd2222836ff8a17c7bfcc7611d2ee2b

The last commit's patch header even references the previous submission
you had for this patch with:

"These changes also stave off the need to introduce new DM-specific
workarounds in block core, e.g. this proposal:
https://patchwork.kernel.org/patch/10067961/"