Message-ID: <66bf85f635b2e_184d66294b9@willemb.c.googlers.com.notmuch>
Date: Fri, 16 Aug 2024 13:01:42 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Joe Damato <jdamato@...tly.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Martin Karsten <mkarsten@...terloo.ca>, 
 Samiullah Khawaja <skhawaja@...gle.com>, 
 Stanislav Fomichev <sdf@...ichev.me>, 
 netdev@...r.kernel.org, 
 amritha.nambiar@...el.com, 
 sridhar.samudrala@...el.com, 
 Alexander Lobakin <aleksander.lobakin@...el.com>, 
 Alexander Viro <viro@...iv.linux.org.uk>, 
 Breno Leitao <leitao@...ian.org>, 
 Christian Brauner <brauner@...nel.org>, 
 Daniel Borkmann <daniel@...earbox.net>, 
 "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Jan Kara <jack@...e.cz>, 
 Jiri Pirko <jiri@...nulli.us>, 
 Johannes Berg <johannes.berg@...el.com>, 
 Jonathan Corbet <corbet@....net>, 
 "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, 
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 Lorenzo Bianconi <lorenzo@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [RFC net-next 0/5] Suspend IRQs during preferred busy poll

Joe Damato wrote:
> On Fri, Aug 16, 2024 at 10:59:51AM -0400, Willem de Bruijn wrote:
> > Willem de Bruijn wrote:
> > > Martin Karsten wrote:
> > > > On 2024-08-14 15:53, Samiullah Khawaja wrote:
> > > > > On Tue, Aug 13, 2024 at 6:19 AM Martin Karsten <mkarsten@...terloo.ca> wrote:
> > > > >>
> > > > >> On 2024-08-13 00:07, Stanislav Fomichev wrote:
> > > > >>> On 08/12, Martin Karsten wrote:
> > > > >>>> On 2024-08-12 21:54, Stanislav Fomichev wrote:
> > > > >>>>> On 08/12, Martin Karsten wrote:
> > > > >>>>>> On 2024-08-12 19:03, Stanislav Fomichev wrote:
> > > > >>>>>>> On 08/12, Martin Karsten wrote:
> > > > >>>>>>>> On 2024-08-12 16:19, Stanislav Fomichev wrote:
> > > > >>>>>>>>> On 08/12, Joe Damato wrote:
> > > > >>>>>>>>>> Greetings:
> > > > 
> > > > [snip]
> > > > 
> > > > >>>>>> Note that napi_suspend_irqs/napi_resume_irqs is needed even for the sake of
> > > > >>>>>> an individual queue or application to make sure that IRQ suspension is
> > > > >>>>>> enabled/disabled right away when the state of the system changes from busy
> > > > >>>>>> to idle and back.
> > > > >>>>>
> > > > >>>>> Can we not handle everything in napi_busy_loop? If we can mark some napi
> > > > >>>>> contexts as "explicitly polled by userspace with a larger defer timeout",
> > > > >>>>> we should be able to do better compared to current NAPI_F_PREFER_BUSY_POLL
> > > > >>>>> which is more like "this particular napi_poll call is user busy polling".
> > > > >>>>
> > > > >>>> Then either the application needs to be polling all the time (wasting cpu
> > > > >>>> cycles) or latencies will be determined by the timeout.
> > > > > But if I understand correctly, this means that if the application
> > > > > thread that is supposed
> > > > > to do napi busy polling gets busy doing work on the new data/events in
> > > > > userspace, napi polling
> > > > > will not be done until the suspend_timeout triggers? Do you dispatch
> > > > > work to a separate worker
> > > > > threads, in userspace, from the thread that is doing epoll_wait?
> > > > 
> > > > Yes, napi polling is suspended while the application is busy between 
> > > > epoll_wait calls. That's where the benefits are coming from.
> > > > 
> > > > The consequences depend on the nature of the application and overall 
> > > > preferences for the system. If there's a "dominant" application for a 
> > > > number of queues and cores, the resulting latency for other background 
> > > > applications using the same queues might not be a problem at all.
> > > > 
> > > > One other simple mitigation is limiting the number of events that each 
> > > > epoll_wait call accepts. Note that this batch size also determines the 
> > > > worst-case latency for the application in question, so there is a 
> > > > natural incentive to keep it limited.
> > > > 
> > > > A more complex application design, like you suggest, might also be an 
> > > > option.
> > > > 
> > > > >>>> Only when switching back and forth between polling and interrupts is it
> > > > >>>> possible to get low latencies across a large spectrum of offered loads
> > > > >>>> without burning cpu cycles at 100%.
> > > > >>>
> > > > >>> Ah, I see what you're saying, yes, you're right. In this case ignore my comment
> > > > >>> about ep_suspend_napi_irqs/napi_resume_irqs.
> > > > >>
> > > > >> Thanks for probing and double-checking everything! Feedback is important
> > > > >> for us to properly document our proposal.
> > > > >>
> > > > >>> Let's see how other people feel about per-dev irq_suspend_timeout. Properly
> > > > >>> disabling napi during busy polling is super useful, but it would still
> > > > >>> be nice to plumb irq_suspend_timeout via epoll context or have it set on
> > > > >>> a per-napi basis imho.
> > > > > I agree, this would allow each napi queue to tune itself based on
> > > > > heuristics. But I think
> > > > > doing it through epoll independent interface makes more sense as Stan
> > > > > suggested earlier.
> > > > 
> > > > The question is whether to add a useful mechanism (one sysfs parameter 
> > > > and a few lines of code) that is optional, but with demonstrable and 
> > > > significant performance/efficiency improvements for an important class 
> > > > of applications - or wait for an uncertain future?
> > > 
> > > The issue is that this one little change can never be removed, as it
> > > becomes ABI.
> > > 
> > > Let's get the right API from the start.
> > > 
> > > Not sure that a global variable, or sysfs as API, is the right one.
> > 
> > Sorry per-device, not global.
> > 
> > My main concern is that it adds yet another user tunable integer, for
> > which the right value is not obvious.
> 
> This is a feature for advanced users just like SO_INCOMING_NAPI_ID
> and countless other features.
> 
> The value may not be obvious, but guidance (in the form of
> documentation) can be provided.

Okay. Could you share a stab at what that would look like?

> > If the only goal is to safely reenable interrupts when the application
> > stops calling epoll_wait, does this have to be user tunable?
> > 
> > Can it be either a single good enough constant, or derived from
> > another tunable, like busypoll_read.
> 
> I believe you meant busy_read here, is that right?
> 
> At any rate:
> 
>   - I don't think a single constant is appropriate, just as it
>     wasn't appropriate for the existing mechanism
>     (napi_defer_hard_irqs/gro_flush_timeout), and
> 
>   - Deriving the value from a pre-existing parameter to preserve the
>     ABI, like busy_read, makes using this more confusing for users
>     and complicates the API significantly.
> 
> I agree we should get the API right from the start; that's why we've
> submit this as an RFC ;)
> 
> We are happy to take suggestions from the community, but, IMHO,
> re-using an existing parameter for a different purpose only in
> certain circumstances (if I understand your suggestions) is a much
> worse choice than adding a new tunable that clearly states its
> intended singular purpose.

Ack. I was thinking whether an epoll flag through your new epoll
ioctl interface to toggle the IRQ suspension (and timer start)
would be preferable. Because more fine grained.

Also, the value is likely dependent more on the expected duration
of userspace processing? If so, it would be the same for all
devices, so does a per-netdev value make sense?

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