Date:   Wed, 20 Feb 2019 15:00:34 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     Ido Schimmel <idosch@...sch.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH net-next v4 07/17] net: sched: protect filter_chain list
 with filter_chain_lock mutex

On Tue, Feb 19, 2019 at 7:20 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>
>
> On Tue 19 Feb 2019 at 05:08, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > On Fri, Feb 15, 2019 at 2:02 AM Vlad Buslov <vladbu@...lanox.com> wrote:
> >>
> >> I looked at the code and problem seems to be matchall classifier
> >> specific. My implementation of unlocked cls API assumes that concurrent
> >> insertions are possible and checks for it when deleting "empty" tp.
> >> Since classifiers don't expose number of elements, the only way to test
> >> this is to do tp->walk() on them and assume that walk callback is called
> >> once per filter on every classifier. In your example new tp is created
> >> for second filter, filter insertion fails, number of elements on newly
> >> created tp is checked with tp->walk() before deleting it. However,
> >> matchall classifier always calls the tp->walk() callback once, even when
> >> it doesn't have a valid filter (in this case with NULL filter pointer).
> >
> > Again, this can be eliminated by just switching to normal
> > non-retry logic. This is yet another headache to review this
> > kind of unlock-and-retry logic, I have no idea why you are such
> > a big fan of it.
>
> The retry approach was suggested to me multiple times by Jiri on
> previous code reviews so I assumed it is preferred approach in such
> cases. I don't have a strong preference in this regard, but locking
> whole tp on filter update will remove any parallelism when updating same
> classifier instance concurrently. The goal of these changes is to allow
> parallel rule update and to achieve that I had to introduce some
> complexity into the code.

Yeah, but with unlock-and-retry it would waste more time when
retry occurs. So it can't be better in the worst scenario.

The question is essentially that do we want to waste CPU cycles
when conflicts occurs or just block there until it is safe to enter
the critical section?

And, is the retry bound? Is it possible that we would retry infinitely
as long as we time it correctly?


>
> Now let me explain why these two approaches result completely different
> performance in this case. Lets start with a list of most CPU-consuming
> parts in new filter creation process in descending order (raw data at
> the end of this mail):
>
> 1) Hardware offload - if available and no skip_hw.
> 2) Exts (actions) initalization - most expensive part even with single
> action, CPU usage increases with number of actions per filter.
> 3) cls API.
> 4) Flower classifier data structure initialization.
>
> Note that 1)+2) is ~80% of cost of creating a flower filter. So if we
> just lock the whole flower classifier instance during rule update we
> serialize 1, 2 and 4, and only cls API (~13% of CPU cost) can be
> executed concurrently. However, in proposed flower implementation hw
> offloading and action initialization code is called without any locks
> and tp->lock is only obtained when modifying flower data structures,
> which means that only 3) is serialized and everything else (87% of CPU
> cost) can be executed in parallel.

What about when conflicts detected and retry the whole change?
And, of course, how often do conflicts happen?

Thanks.

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