linux-kernel - Re: [PATCH net] net: mana: Fix perf regression: remove rx_cqes, tx

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Date:   Fri, 26 May 2023 08:45:31 +0200
From:   Horatiu Vultur <horatiu.vultur@...rochip.com>
To:     Haiyang Zhang <haiyangz@...rosoft.com>
CC:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Paul Rosswurm <paulros@...rosoft.com>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
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        "pabeni@...hat.com" <pabeni@...hat.com>,
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        Long Li <longli@...rosoft.com>,
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        Ajay Sharma <sharmaajay@...rosoft.com>,
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        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH net] net: mana: Fix perf regression: remove rx_cqes,
 tx_cqes counters

The 05/25/2023 14:34, Haiyang Zhang wrote:
> 
> > -----Original Message-----
> > From: Horatiu Vultur <horatiu.vultur@...rochip.com>
> > Sent: Thursday, May 25, 2023 2:49 AM
> > To: Haiyang Zhang <haiyangz@...rosoft.com>
> > Cc: linux-hyperv@...r.kernel.org; netdev@...r.kernel.org; Dexuan Cui
> > <decui@...rosoft.com>; KY Srinivasan <kys@...rosoft.com>; Paul Rosswurm
> > <paulros@...rosoft.com>; olaf@...fle.de; vkuznets@...hat.com;
> > davem@...emloft.net; wei.liu@...nel.org; edumazet@...gle.com;
> > kuba@...nel.org; pabeni@...hat.com; leon@...nel.org; Long Li
> > <longli@...rosoft.com>; ssengar@...ux.microsoft.com; linux-
> > rdma@...r.kernel.org; daniel@...earbox.net; john.fastabend@...il.com;
> > bpf@...r.kernel.org; ast@...nel.org; Ajay Sharma
> > <sharmaajay@...rosoft.com>; hawk@...nel.org; linux-
> > kernel@...r.kernel.org; stable@...r.kernel.org
> > Subject: Re: [PATCH net] net: mana: Fix perf regression: remove rx_cqes,
> > tx_cqes counters
> >
> > [Some people who received this message don't often get email from
> > horatiu.vultur@...rochip.com. Learn why this is important at
> > https://aka.ms/LearnAboutSenderIdentification ]
> >
> > The 05/24/2023 14:22, Haiyang Zhang wrote:
> >
> > Hi Haiyang,
> >
> > >
> > > The apc->eth_stats.rx_cqes is one per NIC (vport), and it's on the
> > > frequent and parallel code path of all queues. So, r/w into this
> > > single shared variable by many threads on different CPUs creates a
> > > lot caching and memory overhead, hence perf regression. And, it's
> > > not accurate due to the high volume concurrent r/w.
> >
> > Do you have any numbers to show the improvement of this change?
> 
> The numbers are not published. The perf regression of the previous
> patch is very significant, and this patch eliminates the regression.
> 
> >
> > >
> > > Since the error path of mana_poll_rx_cq() already has warnings, so
> > > keeping the counter and convert it to a per-queue variable is not
> > > necessary. So, just remove this counter from this high frequency
> > > code path.
> > >
> > > Also, remove the tx_cqes counter for the same reason. We have
> > > warnings & other counters for errors on that path, and don't need
> > > to count every normal cqe processing.
> >
> > Will you not have problems with the counter 'apc->eth_stats.tx_cqe_err'?
> > It is not in the hot path but you will have concurrent access to it.
> 
> Yes, but that error happens rarely, so a shared variable is good enough. So, I
> don't change it in this patch.

OK, I understand.
Maybe this can be fixed in a different patch at a later point. Thanks.

Reviwed-by: Horatiu Vultur <horatiu.vultur@...rochip.com>

> 
> Thanks,
> - Haiyang
> 

-- 
/Horatiu