Re: Regarding patch "block/blk-mq: Don't complete locally if capacities are different"

From: Bart Van Assche
Date: Wed Aug 21 2024 - 13:22:43 EST


On 8/21/24 5:29 AM, MANISH PANDEY wrote:
How about introducing a new rq_affinity ( may be rq_affinity = 3) for using cpus_equal_capacity() using new flag QUEUE_FLAG_SAME_CAPACITY.

if (cpu == rq->mq_ctx->cpu ||
    (!test_bit(QUEUE_FLAG_SAME_FORCE, &rq->q->queue_flags) &&
      cpus_share_cache(cpu, rq->mq_ctx->cpu) &&
+      (test_bit(QUEUE_FLAG_CPU_CAPACITY, &rq->q->queue_flags))
       && cpus_equal_capacity(cpu, rq->mq_ctx->cpu)))
        return false;

Could you please consider raising similar change, if this seems fine for all.

I'm not sure that a change like the above would be acceptable.

What is the performance impact of the above change? Redirecting
completion interrupts from a slow core to a fast core causes additional
cache misses if the I/O was submitted from a slow core. Are there
perhaps use cases for which the above change slows down I/O?

Thanks,

Bart.