Re: [RFC v2 05/83] Add NOVA filesystem definitions and useful helper routines.

Eric Biggers
2018-03-11T19:23:02Z

On Sun, Mar 11, 2018 at 02:00:13PM +0200, Nikolay Borisov wrote:
> [Adding Herbert Xu to CC since he is the maintainer of the crypto subsys
> maintainer]
>
> On 10.03.2018 20:17, Andiry Xu wrote:
> <snip>
>
> > +static inline u32 nova_crc32c(u32 crc, const u8 *data, size_t len)
> > +{
> > + u8 *ptr = (u8 *) data;
> > + u64 acc = crc; /* accumulator, crc32c value in lower 32b */
> > + u32 csum;
> > +
> > + /* x86 instruction crc32 is part of SSE-4.2 */
> > + if (static_cpu_has(X86_FEATURE_XMM4_2)) {
> > + /* This inline assembly implementation should be equivalent
> > + * to the kernel's crc32c_intel_le_hw() function used by
> > + * crc32c(), but this performs better on test machines.
> > + */
> > + while (len > 8) {
> > + asm volatile(/* 64b quad words */
> > + "crc32q (%1), %0"
> > + : "=r" (acc)
> > + : "r" (ptr), "0" (acc)
> > + );
> > + ptr += 8;
> > + len -= 8;
> > + }
> > +
> > + while (len > 0) {
> > + asm volatile(/* trailing bytes */
> > + "crc32b (%1), %0"
> > + : "=r" (acc)
> > + : "r" (ptr), "0" (acc)
> > + );
> > + ptr++;
> > + len--;
> > + }
> > +
> > + csum = (u32) acc;
> > + } else {
> > + /* The kernel's crc32c() function should also detect and use the
> > + * crc32 instruction of SSE-4.2. But calling in to this function
> > + * is about 3x to 5x slower than the inline assembly version on
> > + * some test machines.
>
> That is really odd. Did you try to characterize why this is the case? Is
> it purely the overhead of dispatching to the correct backend function?
> That's a rather big performance hit.
>
> > + */
> > + csum = crc32c(crc, data, len);
> > + }
> > +
> > + return csum;
> > +}
> > +

Are you sure that CONFIG_CRYPTO_CRC32C_INTEL was enabled during your tests and
that the accelerated version was being called? Or, perhaps CRC32C_PCL_BREAKEVEN
(defined in arch/x86/crypto/crc32c-intel_glue.c) needs to be adjusted. Please
don't hack around performance problems like this; if they exist, they need to be
fixed for everyone.

Eric