Messages in this thread | | | Date | Mon, 22 Aug 2022 17:17:00 +0200 | From | Sebastian Andrzej Siewior <> | Subject | Re: [PATCH 9/9] u64_stat: Remove the obsolete fetch_irq() variants |
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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
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