Re: [PATCH 0/2] RISC-V: Probe for misaligned access speed

[Conor Dooley <conor@kernel.org>]

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On Fri, Jun 23, 2023 at 03:20:14PM -0700, Evan Green wrote:
> 
> The current setting for the hwprobe bit indicating misaligned access
> speed is controlled by a vendor-specific feature probe function. This is
> essentially a per-SoC table we have to maintain on behalf of each vendor
> going forward. Let's convert that instead to something we detect at
> runtime.
> 
> We have two assembly routines at the heart of our probe: one that
> does a bunch of word-sized accesses (without aligning its input buffer),
> and the other that does byte accesses. If we can move a larger number of
> bytes using misaligned word accesses than we can with the same amount of
> time doing byte accesses, then we can declare misaligned accesses as
> "fast".
> 
> The tradeoff of reducing this maintenance burden is boot time. We spend
> 4-6 jiffies per core doing this measurement (0-2 on jiffie edge
> alignment, and 4 on measurement). The timing loop was based on
> raid6_choose_gen(), which uses (16+1)*N jiffies (where N is the number
> of algorithms). On my THead C906, I found measurements to be stable
> across several reboots, and looked like this:
> 
> [    0.047582] cpu0: Unaligned word copy 1728 MB/s, byte copy 402 MB/s, misaligned accesses are fast
> 
> I don't have a machine where misaligned accesses are slow, but I'd be
> interested to see the results of booting this series if someone did.

Can you elaborate on "results" please? Otherwise,

[    0.333110] smp: Bringing up secondary CPUs ...
[    0.370794] cpu1: Unaligned word copy 2 MB/s, byte copy 231 MB/s, misaligned accesses are slow
[    0.411368] cpu2: Unaligned word copy 2 MB/s, byte copy 231 MB/s, misaligned accesses are slow
[    0.451947] cpu3: Unaligned word copy 2 MB/s, byte copy 231 MB/s, misaligned accesses are slow
[    0.462628] smp: Brought up 1 node, 4 CPUs

[    0.631464] cpu0: Unaligned word copy 2 MB/s, byte copy 229 MB/s, misaligned accesses are slow

btw, why the mixed usage of "unaligned" and misaligned"?

Cheers,
Conor.

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