On Sat, Apr 18, 2020 at 11:09:21AM +0800, Ming Lei wrote:
> -static bool blk_mq_get_driver_tag(struct request *rq)
> +static bool blk_mq_get_driver_tag(struct request *rq, bool direct_issue)
> {
> struct blk_mq_alloc_data data = {
> .q = rq->q,
> @@ -1054,6 +1054,23 @@ static bool blk_mq_get_driver_tag(struct request *rq)
> data.hctx->tags->rqs[rq->tag] = rq;
> }
> allocated:
> + /*
> + * Add one memory barrier in case that direct issue IO process
> + * is migrated to other CPU which may not belong to this hctx,
> + * so we can order driver tag assignment and checking
> + * BLK_MQ_S_INACTIVE. Otherwise, barrier() is enough given the
> + * two code paths are run on single CPU in case that
> + * BLK_MQ_S_INACTIVE is set.
Please use up all 80 characters for the comments (also elsewhere). That
being said I fail to see what the barrier() as a pure compiler barrier
even buys us here.
> + */
> + if (unlikely(direct_issue && rq->mq_ctx->cpu != raw_smp_processor_id()))
> + smp_mb();
> + else
> + barrier();
> +
> + if (unlikely(test_bit(BLK_MQ_S_INACTIVE, &data.hctx->state))) {
> + blk_mq_put_driver_tag(rq);
> + return false;
> + }
> return rq->tag != -1;
Also if you take my cleanup to patch 2, we could just open code the
direct_issue case in the only caller instead of having the magic in the
common routine.
> + if ((cpumask_next_and(-1, hctx->cpumask, cpu_online_mask) != cpu) ||
> + (cpumask_next_and(cpu, hctx->cpumask, cpu_online_mask)
> + < nr_cpu_ids))
No need for the inner braces. Also in this case I think something like:
if (cpumask_next_and(-1, hctx->cpumask, cpu_online_mask) != cpu ||
cpumask_next_and(cpu, hctx->cpumask, cpu_online_mask) < nr_cpu_ids)
might be a tad more readable, but then again this might even be worth
a little inline helper once we start bike shedding.
> + /*
> + * The current CPU is the last one in this hctx, S_INACTIVE
> + * can be observed in dispatch path without any barrier needed,
> + * cause both are run on one same CPU.
> + */
> + set_bit(BLK_MQ_S_INACTIVE, &hctx->state);
> + /*
> + * Order setting BLK_MQ_S_INACTIVE and checking rq->tag & rqs[tag],
> + * and its pair is the smp_mb() in blk_mq_get_driver_tag
> + */
> + smp_mb();
> + blk_mq_hctx_drain_inflight_rqs(hctx);
> + return 0;
FYI, Documentation/core-api/atomic_ops.rst asks for using
smp_mb__before_atomic / smp_mb__after_atomic around the bitops.
> +static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
> +{
> + struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
> + struct blk_mq_hw_ctx, cpuhp_online);
> +
> + if (!cpumask_test_cpu(cpu, hctx->cpumask))
> + return 0;
> +
> + clear_bit(BLK_MQ_S_INACTIVE, &hctx->state);
> return 0;
> }
Why not simply:
if (cpumask_test_cpu(cpu, hctx->cpumask))
clear_bit(BLK_MQ_S_INACTIVE, &hctx->state);
return 0;
Conceptually the changes look fine.
