From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Thu, 6 Sep 2018 20:57:01 +0800 From: Ming Lei To: "jianchao.wang" Cc: axboe@kernel.dk, linux-block@vger.kernel.org, jsmart2021@gmail.com, sagi@grimberg.me, linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, keith.busch@intel.com, jthumshirn@suse.de, bart.vanassche@wdc.com Subject: Re: [PATCH 0/3] Introduce a light-weight queue close feature Message-ID: <20180906125659.GA31250@ming.t460p> References: <1536120586-3378-1-git-send-email-jianchao.w.wang@oracle.com> <20180905212659.GB21352@ming.t460p> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: List-ID: On Thu, Sep 06, 2018 at 09:51:43AM +0800, jianchao.wang wrote: > Hi Ming > > On 09/06/2018 05:27 AM, Ming Lei wrote: > > On Wed, Sep 05, 2018 at 12:09:43PM +0800, Jianchao Wang wrote: > >> Dear all > >> > >> As we know, queue freeze is used to stop new IO comming in and drain > >> the request queue. And the draining queue here is necessary, because > >> queue freeze kills the percpu-ref q_usage_counter and need to drain > >> the q_usage_counter before switch it back to percpu mode. This could > >> be a trouble when we just want to prevent new IO. > >> > >> In nvme-pci, nvme_dev_disable freezes queues to prevent new IO. > >> nvme_reset_work will unfreeze and wait to drain the queues. However, > >> if IO timeout at the moment, no body could do recovery as nvme_reset_work > >> is waiting. We will encounter IO hang. > > > > As we discussed this nvme time issue before, I have pointed out that > > this is because of blk_mq_unfreeze_queue()'s limit which requires that > > unfreeze can only be done when this queue ref counter drops to zero. > > > > For this nvme timeout case, we may relax the limit, for example, > > introducing another API of blk_freeze_queue_stop() as counter-pair of > > blk_freeze_queue_start(), and simply switch the percpu-ref to percpu mode > > from atomic mode inside the new API. > > Looks like we cannot switch a percpu-ref to percpu mode directly w/o drain it. > Some references maybe lost. > > static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref) > { > unsigned long __percpu *percpu_count = percpu_count_ptr(ref); > int cpu; > > BUG_ON(!percpu_count); > > if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC)) > return; > > atomic_long_add(PERCPU_COUNT_BIAS, &ref->count); > > /* > * Restore per-cpu operation. smp_store_release() is paired > * with READ_ONCE() in __ref_is_percpu() and guarantees that the > * zeroing is visible to all percpu accesses which can see the > * following __PERCPU_REF_ATOMIC clearing. > */ > for_each_possible_cpu(cpu) > *per_cpu_ptr(percpu_count, cpu) = 0; > > smp_store_release(&ref->percpu_count_ptr, > ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC); Before REF_ATOMIC is cleared, all counting is done on the atomic type of &ref->count, and it is easy to keep the reference counter at ATOMIC mode. Also the reference counter can only be READ at atomic mode. So could you explain a bit how the lost may happen? And it is lost at atomic mode or percpu mode? > } > > > > >> > >> So introduce a light-weight queue close feature in this patch set > >> which could prevent new IO and needn't drain the queue. > > > > Frankly speaking, IMO, it may not be an good idea to mess up the fast path > > just for handling the extremely unusual timeout event. The same is true > > for doing the preemp only stuff, as you saw I have posted patchset for > > killing it. > > > > In normal case, it is just a judgment like > > if (unlikely(READ_ONCE(q->queue_gate)) > > It should not be a big deal. Adding this stuff in fast path is quite difficult to verify its correctness because it is really lockless, or even barrier-less. Not to mention, READ_ONCE() implies one barrier of smp_read_barrier_depends(). Thanks, Ming