linux-kernel - RE: [PATCH] riscv: use the generic string routines

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Date:   Thu, 5 Aug 2021 08:20:15 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Palmer Dabbelt' <palmer@...belt.com>,
        "mcroce@...ux.microsoft.com" <mcroce@...ux.microsoft.com>,
        "mcroce@...ux.microsoft.com" <mcroce@...ux.microsoft.com>
CC:     "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        Atish Patra <Atish.Patra@....com>,
        "kernel@...il.dk" <kernel@...il.dk>,
        "akira.tsukamoto@...il.com" <akira.tsukamoto@...il.com>,
        "drew@...gleboard.org" <drew@...gleboard.org>,
        "bmeng.cn@...il.com" <bmeng.cn@...il.com>,
        "guoren@...nel.org" <guoren@...nel.org>,
        "Christoph Hellwig" <hch@...radead.org>
Subject: RE: [PATCH] riscv: use the generic string routines

From: Palmer Dabbelt
> Sent: 04 August 2021 21:40
> 
> On Tue, 03 Aug 2021 09:54:34 PDT (-0700), mcroce@...ux.microsoft.com wrote:
> > On Mon, Jul 19, 2021 at 1:44 PM Matteo Croce <mcroce@...ux.microsoft.com> wrote:
> >>
> >> From: Matteo Croce <mcroce@...rosoft.com>
> >>
> >> Use the generic routines which handle alignment properly.
> >>
> >> These are the performances measured on a BeagleV machine for a
> >> 32 mbyte buffer:
> >>
> >> memcpy:
> >> original aligned:        75 Mb/s
> >> original unaligned:      75 Mb/s
> >> new aligned:            114 Mb/s
> >> new unaligned:          107 Mb/s
> >>
> >> memset:
> >> original aligned:       140 Mb/s
> >> original unaligned:     140 Mb/s
> >> new aligned:            241 Mb/s
> >> new unaligned:          241 Mb/s
> >>
> >> TCP throughput with iperf3 gives a similar improvement as well.
> >>
> >> This is the binary size increase according to bloat-o-meter:
> >>
> >> add/remove: 0/0 grow/shrink: 4/2 up/down: 432/-36 (396)
> >> Function                                     old     new   delta
> >> memcpy                                        36     324    +288
> >> memset                                        32     148    +116
> >> strlcpy                                      116     132     +16
> >> strscpy_pad                                   84      96     +12
> >> strlcat                                      176     164     -12
> >> memmove                                       76      52     -24
> >> Total: Before=1225371, After=1225767, chg +0.03%
> >>
> >> Signed-off-by: Matteo Croce <mcroce@...rosoft.com>
> >> Signed-off-by: Emil Renner Berthing <kernel@...il.dk>
> >> ---
> >
> > Hi,
> >
> > can someone have a look at this change and share opinions?
> 
> This LGTM.  How are the generic string routines landing?  I'm happy to
> take this into my for-next, but IIUC we need the optimized generic
> versions first so we don't have a performance regression falling back to
> the trivial ones for a bit.  Is there a shared tag I can pull in?

I thought the actual problem was that the asm copy functions were
doing misaligned transfers and faulting.

There is no way that the simple C loop should be as fast as
the asm function given the delay cycles reading from memory.

You definitely need to test much smaller copies where the
buffers are resident in the L1 data cache.
Anything else is completely dominated by the cache line fills/spills.

You also need to test on the much faster riscv implementations
not just on the beaglev board.

	David

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