linux-kernel - Re: [PATCH 7/8] wbt: add general throttling mechanism

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Date:	Wed, 27 Apr 2016 20:06:57 +0800
From:	xiakaixu <xiakaixu@...wei.com>
To:	Jens Axboe <axboe@...com>
CC:	<linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-block@...r.kernel.org>, <jack@...e.cz>,
	<dchinner@...hat.com>, <sedat.dilek@...il.com>,
	"miaoxie (A)" <miaoxie@...wei.com>, Huxinwei <huxinwei@...wei.com>,
	Bintian <bintian.wang@...wei.com>
Subject: Re: [PATCH 7/8] wbt: add general throttling mechanism


> +	return rwb && rwb->wb_normal != 0;
> +}
> +
> +/*
> + * Increment 'v', if 'v' is below 'below'. Returns true if we succeeded,
> + * false if 'v' + 1 would be bigger than 'below'.
> + */
> +static bool atomic_inc_below(atomic_t *v, int below)
> +{
> +	int cur = atomic_read(v);
> +
> +	for (;;) {
> +		int old;
> +
> +		if (cur >= below)
> +			return false;
> +		old = atomic_cmpxchg(v, cur, cur + 1);
> +		if (old == cur)
> +			break;
> +		cur = old;
> +	}
> +
> +	return true;
> +}
> +
> +static void wb_timestamp(struct rq_wb *rwb, unsigned long *var)
> +{
> +	if (rwb_enabled(rwb)) {
> +		const unsigned long cur = jiffies;
> +
> +		if (cur != *var)
> +			*var = cur;
> +	}
> +}
> +
> +void __wbt_done(struct rq_wb *rwb)
> +{
> +	int inflight, limit = rwb->wb_normal;
> +
> +	/*
> +	 * If the device does write back caching, drop further down
> +	 * before we wake people up.
> +	 */
> +	if (rwb->wc && !atomic_read(&rwb->bdi->wb.dirty_sleeping))
> +		limit = 0;
> +	else
> +		limit = rwb->wb_normal;
> +
> +	/*
> +	 * Don't wake anyone up if we are above the normal limit. If
> +	 * throttling got disabled (limit == 0) with waiters, ensure
> +	 * that we wake them up.
> +	 */
> +	inflight = atomic_dec_return(&rwb->inflight);
> +	if (limit && inflight >= limit) {
> +		if (!rwb->wb_max)
> +			wake_up_all(&rwb->wait);
> +		return;
> +	}
> +
Hi Jens,

Just a little confused about this. The rwb->wb_max can't be 0 if the variable
'limit' does not equal to 0. So the if (!rwb->wb_max) branch maybe does not
make sense.


> +	if (waitqueue_active(&rwb->wait)) {
> +		int diff = limit - inflight;
> +
> +		if (!inflight || diff >= rwb->wb_background / 2)
> +			wake_up_nr(&rwb->wait, 1);
> +	}
> +}
> +
> +/*
> + * Called on completion of a request. Note that it's also called when
> + * a request is merged, when the request gets freed.
> + */
> +void wbt_done(struct rq_wb *rwb, struct wb_issue_stat *stat)
> +{
> +	if (!rwb)
> +		return;
> +
> +	if (!wbt_tracked(stat)) {
> +		if (rwb->sync_cookie == stat) {
> +			rwb->sync_issue = 0;
> +			rwb->sync_cookie = NULL;
> +		}
> +
> +		wb_timestamp(rwb, &rwb->last_comp);
> +	} else {
> +		WARN_ON_ONCE(stat == rwb->sync_cookie);
> +		__wbt_done(rwb);
> +		wbt_clear_tracked(stat);
> +	}
> +}
> +
> +static void calc_wb_limits(struct rq_wb *rwb)
> +{
> +	unsigned int depth;
> +
> +	if (!rwb->min_lat_nsec) {
> +		rwb->wb_max = rwb->wb_normal = rwb->wb_background = 0;
> +		return;
> +	}
> +
> +	depth = min_t(unsigned int, RWB_MAX_DEPTH, rwb->queue_depth);
> +
> +	/*
> +	 * Reduce max depth by 50%, and re-calculate normal/bg based on that
> +	 */
> +	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> +	rwb->wb_normal = (rwb->wb_max + 1) / 2;
> +	rwb->wb_background = (rwb->wb_max + 3) / 4;
> +}
> +
> +static bool inline stat_sample_valid(struct blk_rq_stat *stat)
> +{
> +	/*
> +	 * We need at least one read sample, and a minimum of
> +	 * RWB_MIN_WRITE_SAMPLES. We require some write samples to know
> +	 * that it's writes impacting us, and not just some sole read on
> +	 * a device that is in a lower power state.
> +	 */
> +	return stat[0].nr_samples >= 1 &&
> +		stat[1].nr_samples >= RWB_MIN_WRITE_SAMPLES;
> +}
> +