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From: Andrea Parri <parri.andrea@gmail.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Michael Kelley <mikelley@microsoft.com>,
	Dexuan Cui <decui@microsoft.com>,
	KY Srinivasan <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Wei Liu <wei.liu@kernel.org>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 02/11] Drivers: hv: vmbus: Don't bind the offer&rescind works to a specific CPU
Date: Fri, 3 Apr 2020 14:04:56 +0200
Message-ID: <20200403120456.GA24298@andrea> (raw)
In-Reply-To: <87o8se2fpr.fsf@vitty.brq.redhat.com>

On Mon, Mar 30, 2020 at 02:24:16PM +0200, Vitaly Kuznetsov wrote:
> Michael Kelley <mikelley@microsoft.com> writes:
> 
> > From: Andrea Parri <parri.andrea@gmail.com> Sent: Saturday, March 28, 2020 10:09 AM
> >> 
> >> > In case we believe that OFFER -> RESCINF sequence is always ordered
> >> > by the host AND we don't care about other offers in the queue the
> >> > suggested locking is OK: we're guaranteed to process RESCIND after we
> >> > finished processing OFFER for the same channel. However, waiting for
> >> > 'offer_in_progress == 0' looks fishy so I'd suggest we at least add a
> >> > comment explaining that the wait is only needed to serialize us with
> >> > possible OFFER for the same channel - and nothing else. I'd personally
> >> > still slightly prefer the algorythm I suggested as it guarantees we take
> >> > channel_mutex with offer_in_progress == 0 -- even if there are no issues
> >> > we can think of today (not strongly though).
> >> 
> >> Does it?  offer_in_progress is incremented without channel_mutex...
> >> 
> 
> No, it does not, you're right, by itself the change is insufficient.
> 
> >> IAC, I have no objections to apply the changes you suggested.  To avoid
> >> misunderstandings: vmbus_bus_suspend() presents a similar usage...  Are
> >> you suggesting that I apply similar changes there?
> >> 
> >> Alternatively:  FWIW, the comment in vmbus_onoffer_rescind() does refer
> >> to "The offer msg and the corresponding rescind msg...".  I am all ears
> >> if you have any concrete suggestions to improve these comments.
> >> 
> >
> > Given that waiting for 'offer_in_progress == 0' is the current code, I think
> > there's an argument to made for not changing it if the change isn't strictly
> > necessary.  This patch set introduces enough change that *is* necessary. :-)
> >
> 
> Sure. I was thinking a bit more about this and it seems that over years
> we've made the synchronization of channels code too complex (every time
> for a good reason but still). Now (before this series) we have at least:
> 
> vmbus_connection.channel_mutex
> vmbus_connection.offer_in_progress
> channel.probe_done
> channel.rescind
> Workqueues (vmbus_connection.work_queue,
>  queue_work_on(vmbus_connection.connect_cpu),...)
> channel.lock spinlock (the least of the problems)
> 
> Maybe there's room for improvement? Out of top of my head I'd suggest a
> state machine for each channel (e.g something like
> OFFERED->OPENING->OPEN->RESCIND_REQ->RESCINDED->CLOSED) + refcounting
> (subchannels, open/rescind/... requests in progress, ...) + non-blocking
> request handling like "Can we handle this rescind offer now? No,
> refcount is too big. OK, rescheduling the work". Maybe not the best
> design ever and I'd gladly support any other which improves the
> readability of the code and makes all state changes and synchronization
> between them more obvious.
> 
> Note, VMBus channel handling driven my messages (unlike events for ring
> buffer) is not performance critical, we just need to ensure completeness
> (all requests are handled correctly) with forward progress guarantees
> (no deadlocks).
> 
> I understand the absence of 'hot' issues in the current code is what can
> make the virtue of redesign questionable and sorry for hijacking the
> series which doesn't seem to make things worse :-)

(Back here...  Sorry for the delay.)

FWIW, what you wrote above makes sense to me; and *yes*, the series in
question was not intended as "let us undertake such a redesign" (quite
the opposite in fact...)

With regard to this specific patch, it seems to me that we've reached
a certain consensus or, at least, I don't see complaints  ;-).  Please
let me know if I misunderstood.

Thanks,
  Andrea

  reply index

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-25 22:54 [RFC PATCH 00/11] VMBus channel interrupt reassignment Andrea Parri (Microsoft)
2020-03-25 22:54 ` [RFC PATCH 01/11] Drivers: hv: vmbus: Always handle the VMBus messages on CPU0 Andrea Parri (Microsoft)
2020-03-26 14:05   ` Vitaly Kuznetsov
2020-03-28 18:50     ` Andrea Parri
2020-03-25 22:54 ` [RFC PATCH 02/11] Drivers: hv: vmbus: Don't bind the offer&rescind works to a specific CPU Andrea Parri (Microsoft)
2020-03-26 14:16   ` Vitaly Kuznetsov
2020-03-26 15:47     ` Andrea Parri
2020-03-26 17:26       ` Vitaly Kuznetsov
2020-03-28 17:08         ` Andrea Parri
2020-03-29  3:43           ` Michael Kelley
2020-03-30 12:24             ` Vitaly Kuznetsov
2020-04-03 12:04               ` Andrea Parri [this message]
2020-03-25 22:54 ` [RFC PATCH 03/11] Drivers: hv: vmbus: Replace the per-CPU channel lists with a global array of channels Andrea Parri (Microsoft)
2020-03-26 14:31   ` Vitaly Kuznetsov
2020-03-26 17:05     ` Andrea Parri
2020-03-26 17:43       ` Vitaly Kuznetsov
2020-03-28 18:21         ` Andrea Parri
2020-03-29  3:49           ` Michael Kelley
2020-03-30 12:45           ` Vitaly Kuznetsov
2020-04-03 13:38             ` Andrea Parri
2020-04-03 14:56               ` Vitaly Kuznetsov
2020-03-25 22:54 ` [RFC PATCH 04/11] hv_netvsc: Disable NAPI before closing the VMBus channel Andrea Parri (Microsoft)
2020-03-26 15:26   ` Stephen Hemminger
2020-03-26 17:55     ` Andrea Parri
2020-03-25 22:54 ` [RFC PATCH 05/11] hv_utils: Always execute the fcopy and vss callbacks in a tasklet Andrea Parri (Microsoft)
2020-03-25 22:55 ` [RFC PATCH 06/11] Drivers: hv: vmbus: Use a spin lock for synchronizing channel scheduling vs. channel removal Andrea Parri (Microsoft)
2020-03-25 22:55 ` [RFC PATCH 07/11] PCI: hv: Prepare hv_compose_msi_msg() for the VMBus-channel-interrupt-to-vCPU reassignment functionality Andrea Parri (Microsoft)
2020-03-25 22:55 ` [RFC PATCH 08/11] Drivers: hv: vmbus: Remove the unused HV_LOCALIZED channel affinity logic Andrea Parri (Microsoft)
2020-03-25 22:55 ` [RFC PATCH 09/11] Drivers: hv: vmbus: Synchronize init_vp_index() vs. CPU hotplug Andrea Parri (Microsoft)
2020-03-25 22:55 ` [RFC PATCH 10/11] Drivers: hv: vmbus: Introduce the CHANNELMSG_MODIFYCHANNEL message type Andrea Parri (Microsoft)
2020-03-26 14:46   ` Vitaly Kuznetsov
2020-03-28 18:48     ` Andrea Parri
2020-04-03 14:55       ` Andrea Parri
2020-03-25 22:55 ` [RFC PATCH 11/11] scsi: storvsc: Re-init stor_chns when a channel interrupt is re-assigned Andrea Parri (Microsoft)
2020-03-30 16:42   ` Michael Kelley
2020-03-30 18:55     ` Andrea Parri
2020-03-30 19:49       ` Michael Kelley
2020-04-03 13:41         ` Andrea Parri

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