hi,
On 5/13/20 6:37 AM, Xiaoguang Wang wrote:
When IORING_SETUP_SQPOLL is enabled, io_ring_ctx_wait_and_kill() will wait
for sq thread to idle by busy loop:
while (ctx->sqo_thread && !wq_has_sleeper(&ctx->sqo_wait))
cond_resched();
Above codes are not friendly, indeed I think this busy loop will introduce a
cpu burst in current cpu, though it maybe short.
In this patch, if ctx->refs is dying, we forbids sq_thread from submitting
sqes anymore, just discard leftover sqes.
I don't think this really changes anything. What happens if:
@@ -6051,7 +6053,8 @@ static int io_sq_thread(void *data)
}
mutex_lock(&ctx->uring_lock);
- ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
+ if (likely(!percpu_ref_is_dying(&ctx->refs)))
+ ret = io_submit_sqes(ctx, to_submit, NULL, -1, true);
mutex_unlock(&ctx->uring_lock);
timeout = jiffies + ctx->sq_thread_idle;
You check for dying here, but that could change basically while you're
checking it. So you're still submitting sqes with a ref that's going
away. You've only reduced the window, you haven't eliminated it.
Look at codes, we call percpu_ref_kill() under uring_lock, so isn't it safe
to check the refs' dying status? Thanks.
Regards,
Xiaouang Wang
