linux-kernel - Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme

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Date:   Tue, 12 Dec 2017 11:09:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.org, oleg@...hat.com,
        kernel-team@...com, osandov@...com
Subject: Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU
 and generation based scheme

I don't yet have a full picture, but let me make a few comments.

On Sat, Dec 09, 2017 at 11:25:21AM -0800, Tejun Heo wrote:

> +static u64 blk_mq_rq_aborted_gstate(struct request *rq)
> +{
> +	unsigned int start;
> +	u64 aborted_gstate;
> +
> +	do {
> +		start = u64_stats_fetch_begin(&rq->aborted_gstate_sync);
> +		aborted_gstate = rq->aborted_gstate;
> +	} while (u64_stats_fetch_retry(&rq->aborted_gstate_sync, start));
> +
> +	return aborted_gstate;
> +}

> +	/* if in-flight && overdue, mark for abortion */
> +	if ((gstate & MQ_RQ_STATE_MASK) == MQ_RQ_IN_FLIGHT &&
> +	    time_after_eq(jiffies, deadline)) {
> +		u64_stats_update_begin(&rq->aborted_gstate_sync);
> +		rq->aborted_gstate = gstate;
> +		u64_stats_update_end(&rq->aborted_gstate_sync);
> +		data->nr_expired++;
> +		hctx->nr_expired++;
>  	} else if (!data->next_set || time_after(data->next, deadline)) {

> +	/*
> +	 * ->aborted_gstate is used by the timeout to claim a specific
> +	 * recycle instance of this request.  See blk_mq_timeout_work().
> +	 */
> +	struct u64_stats_sync aborted_gstate_sync;
> +	u64 aborted_gstate;

So I dislike that u64_stats_sync thingy. Esp when used on a single
variable like this.

There are two alternatives, but I don't understand the code well enough
to judge the trade-offs.

1) use gstate_seq for this too; yes it will add some superfluous
   instructions on 64bit targets, but if timeouts are a slow path
   this might not matter.

2) use the pattern we use for cfs_rq::min_vruntime; namely:

	u64 aborted_gstate
#ifdef CONFIG_64BIT
	u64 aborted_gstate_copy;
#endif


static inline void blk_mq_rq_set_abort(struct rq *rq, u64 gstate)
{
	rq->aborted_gstate = gstate;
#ifdef CONFIG_64BIT
	smp_wmb();
	rq->aborted_gstate_copy = gstate;
#endif
}

static inline u64 blk_mq_rq_get_abort(struct rq *rq)
{
#ifdef CONFIG_64BIT
	u64 abort, copy;

	do {
		copy = rq->aborted_gstate_copy;
		smp_rmb();
		abort = rq->aborted_gstate;
	} while (abort != copy);

	return abort;
#else
	return rq->aborted_gstate;
#endif
}

   which is actually _faster_ than the u64_stats_sync stuff (for a
   single variable).


But it might not matter; I just dislike that thing, could be me.