linux-kernel - Re: [PATCH 3/3] scsi/fcoe: Add a local_lock to percpu localport statistics

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Message-ID: <20211117083450.ccgbcmgtwmbkqiqy@linutronix.de>
Date:   Wed, 17 Nov 2021 09:34:50 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     martin.petersen@...cle.com, jejb@...ux.ibm.com, hare@...e.de,
        tglx@...utronix.de, linux-scsi@...r.kernel.org,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 3/3] scsi/fcoe: Add a local_lock to percpu localport
 statistics

On 2021-11-16 18:59:56 [-0800], Davidlohr Bueso wrote:
> Updating the per-CPU lport->stats relies on disabling preemption
> with get/put_cpu() semantics. However, this is a bit harsh for
> PREEMPT_RT and by using a local_lock, it can allow the region
> to be preemptible, guaranteeing CPU locality, without making any
> difference to the non-RT common case as it will continue to
> disable preemption.

What about adding a struct u64_stats_sync where the stats are updated.
You have to use u64_stats_update_begin() at the beginning and the
matching end function instead the get_cpu()/ local_lock().
You need just ensure that there is only one writer at a time. Network
wise it is easy as there are often per-queue stats so one writer at a
time :)

Looking closer, the current approach appears broken in that regard:
stats members, such as TxFrames/ TxWords which are incremented in
bnx2fc_xmit(), are 64bit. Reading/ writing them requires two operations
on 32bit architectures. But we probably don't care because stats and it
happens hardly and performance of course. This is the cute part about
u64_stats_sync, it magically vanishes on 64bit architectures.

Sebastian