linux-kernel - Re: [PATCH 9/9] u64_stat: Remove the obsolete fetch

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Date:   Mon, 22 Aug 2022 17:17:00 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>, netdev@...r.kernel.org
Subject: Re: [PATCH 9/9] u64_stat: Remove the obsolete fetch_irq() variants

On 2022-08-18 10:45:05 [-0700], Jakub Kicinski wrote:
> BTW, I have a hazy memory of people saying they switched to the _irq()
> version because it reduced the number of retries significantly under
> traffic. Dunno how practical that is but would you be willing to scan
> the git history to double check that's not some of the motivation?

I didn't find any changes like -u64_stats_fetch_begin()
+u64_stats_fetch_begin_irq() for perf reasons. Thinking about it:

- On 32bit UP u64_stats_fetch_begin() disables only preemption during
  stats read up, so the number of retries must be 0 because no seqcount
  is used.

- On 32bit UP u64_stats_fetch_begin_irq() disables interrupt during stats
  read out so the number of retries must be 0 because no seqcount is
  used.

- On 32bit SMP u64_stats_fetch_begin()/ u64_stats_fetch_begin_irq()
  keep preemption/ interrupts as-is, uses seqcount and so retries might
  be observed.

Based on that I don't see how a switch to _irq() makes a change in an
SMP build while on UP you shouldn't see a retry at all.

Using u64_stats_fetch_begin() is most likely wrong in networking because
the stats are updated while packets are received which is either in IRQ
or in BH (except maybe for the SPI/I2C ethernet driver). The _bh()
version was replaced with _irq() for netpoll reasons.
u64_stats_fetch_begin() does not disable any of those two (IRQ, BH) on
32bit-UP and does not use a seqcount so it is possible that an update
happens during the read out of stats. Let me look…
	drivers/net/ethernet/cortina/gemini.c
reads in net_device_ops::ndo_get_stats64(), updates in NAPI.

	drivers/net/ethernet/huawei/hinic/hinic_rx.c
reads in net_device_ops::ndo_get_stats64() and
ethtool_ops::get_ethtool_stats(), updates in NAPI. Good.

So I guess these want be fixed properly instead of silently via the
series?

Sebastian