netdev - Re: [PATCH net-next] net: sched: don't release block->lock when dumping chains

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Date:   Tue, 26 Feb 2019 16:09:58 +0000
From:   Vlad Buslov <vladbu@...lanox.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
CC:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next] net: sched: don't release block->lock when
 dumping chains


On Tue 26 Feb 2019 at 00:15, Cong Wang <xiyou.wangcong@...il.com> wrote:
> On Mon, Feb 25, 2019 at 7:45 AM Vlad Buslov <vladbu@...lanox.com> wrote:
>>
>> Function tc_dump_chain() obtains and releases block->lock on each iteration
>> of its inner loop that dumps all chains on block. Outputting chain template
>> info is fast operation so locking/unlocking mutex multiple times is an
>> overhead when lock is highly contested. Modify tc_dump_chain() to only
>> obtain block->lock once and dump all chains without releasing it.
>>
>> Signed-off-by: Vlad Buslov <vladbu@...lanox.com>
>> Suggested-by: Cong Wang <xiyou.wangcong@...il.com>
>
> Thanks for the followup!
>
> Isn't it similar for __tcf_get_next_proto() in tcf_chain_dump()?
> And for tc_dump_tfilter()?

Not really. These two dump all tp filters and not just a template, which
is O(n) on number of filters and can be slow because it calls hw offload
API for each of them. Our typical use-case involves periodic filter dump
(to update stats) while multiple concurrent user-space threads are
updating filters, so it is important for them to be able to execute in
parallel.