On Tue, 22 Sep 2020 at 10:26, David Laight <david.lai...@aculab.com> wrote: > > From: Douglas Anderson > > Sent: 22 September 2020 01:26 > > > > On every boot time we see messages like this: > > > > [ 0.025360] calling calibrate_xor_blocks+0x0/0x134 @ 1 > > [ 0.025363] xor: measuring software checksum speed > > [ 0.035351] 8regs : 3952.000 MB/sec > > [ 0.045384] 32regs : 4860.000 MB/sec > > [ 0.055418] arm64_neon: 5900.000 MB/sec > > [ 0.055423] xor: using function: arm64_neon (5900.000 MB/sec) > > [ 0.055433] initcall calibrate_xor_blocks+0x0/0x134 returned 0 after > > 29296 usecs > > > > As you can see, we spend 30 ms on every boot re-confirming that, yet > > again, the arm64_neon implementation is the fastest way to do XOR. > > ...and the above is on a system with HZ=1000. Due to the way the > > testing happens, if we have HZ defined to something slower it'll take > > much longer. HZ=100 means we spend 300 ms on every boot re-confirming > > a fact that will be the same for every bootup. > > Can't the code use a TSC (or similar high-res counter) to > see how long it takes to process a short 'hot cache' block? > That wouldn't take long at all. >
This is generic code that runs from an core_initcall() so I am not sure we can easily implement this in a portable way. Doug: would it help if we deferred this until late_initcall()? We could take an arbitrary pick from the list at core_initcall() time to serve early users, and update to the fastest one at a later time.