On Wed, Sep 22, 2010 at 12:55:07PM +1000, NeilBrown wrote:
> We limit the number of 'defer' requests to DFR_MAX.
>
> The imposition of this limit is spread about a bit - sometime we don't
> add new things to the list, sometimes we remove old things.
>
> Also it is currently applied to requests which we are 'waiting' for
> rather than 'deferring'. This doesn't seem ideal as 'waiting'
> requests are naturally limited by the number of threads.
>
> So gather the DFR_MAX handling code to one place and only apply it to
> requests that are actually being deferred.
>
> This means that not all 'cache_deferred_req' structures go on the
> 'cache_defer_list, so we need to be careful when removing things.
The idea sounds OK.
> +static void cache_limit_defers(struct cache_deferred_req *dreq)
> +{
> + /* Add 'dreq' to the list of deferred requests and make sure
> + * we don't exceed the limit of allowed deferred requests.
> + */
> + struct cache_deferred_req *discard = NULL;
> +
> + spin_lock(&cache_defer_lock);
> + if (!list_empty(&dreq->recent) ||
> + hlist_unhashed(&dreq->hash)) {
> + /* Must have lost a race, maybe cache_revisit_request is
> + * already processing this. In any case, there is nothing for
> + * us to do.
> + */
> + spin_unlock(&cache_defer_lock);
Did you mean to return here?
--b.
> + }
> +
> + /* First, add this to the list */
> + cache_defer_cnt++;
> + list_add(&dreq->recent, &cache_defer_list);
> +
> + /* now consider removing either the first or the last */
> + if (cache_defer_cnt > DFR_MAX) {
> + if (net_random() & 1)
> + discard = list_entry(cache_defer_list.next,
> + struct cache_deferred_req, recent);
> + else
> + discard = list_entry(cache_defer_list.prev,
> + struct cache_deferred_req, recent);
> + __unhash_deferred_req(discard);
> + }
> + spin_unlock(&cache_defer_lock);
> + if (discard)
> + discard->revisit(discard, 1);
> +}
> +
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