linux-kernel - Re: [PATCH 3/3] powerpc/qspinlock: Use generic smp_cond_load

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Date:   Thu, 18 Mar 2021 13:02:33 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     benh@...nel.crashing.org, Davidlohr Bueso <dbueso@...e.de>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        longman@...hat.com, mingo@...hat.com, mpe@...erman.id.au,
        paulus@...ba.org, peterz@...radead.org, will@...nel.org
Subject: Re: [PATCH 3/3] powerpc/qspinlock: Use generic smp_cond_load_relaxed

On Tue, 16 Mar 2021, Nicholas Piggin wrote:

>One request, could you add a comment in place that references
>smp_cond_load_relaxed() so this commit can be found again if
>someone looks at it? Something like this
>
>/*
> * smp_cond_load_relaxed was found to have performance problems if
> * implemented with spin_begin()/spin_end().
> */

Sure, let me see where I can fit that in and send out a v2.

Similarly, but unrelated to this patch, is there any chance we could
remove the whole spin_until_cond() machinery and make it specific to
powerpc? This was introduced in 2017 and doesn't really have any users
outside of powerpc, except for these:

drivers/firmware/arm_scmi/driver.c:             spin_until_cond(scmi_xfer_done_no_timeout(cinfo, xfer, stop));
drivers/firmware/arm_scmi/shmem.c:      spin_until_cond(ioread32(&shmem->channel_status) &
drivers/net/ethernet/xilinx/ll_temac_main.c:    spin_until_cond(hard_acs_rdy_or_timeout(lp, timeout));

... which afaict only the xilinx one can actually build on powerpc.
Regardless, these could be converted to smp_cond_load_relaxed(), being
the more standard way to do optimized busy-waiting, caring more about
the family of barriers than ad-hoc SMT priorities. Of course, I have
no way of testing any of these changes.

>I wonder if it should have a Fixes: tag to the original commit as
>well.

I'm not sure either. I've actually been informed recently of other
workloads that benefit from the revert on large Power9 boxes. So I'll
go ahead and add it.

>
>Otherwise,
>
>Acked-by: Nicholas Piggin <npiggin@...il.com>

Thanks,
Davidlohr