On 01/09/2019 01:24 PM, Matthew Wilcox wrote: > On Wed, Jan 09, 2019 at 01:03:33PM -0500, Waiman Long wrote: >> The paragraph above may be a bit misleading. This v2 patch actually >> touches very little on percpu accounting aspect of the IRQ counts. See >> patches 2 and 3 for the relevant changes which is just a few line of new >> codes. Please review the individual patches before Nak'ing. >> >> I could theoretically generalize them into a new set of percpu counting >> helpers, but the idea behind it is quite different from the use cases of >> percpu counter. So it may not be a good idea of adding it to there. > Did you even try just using the general purpose infrastructure that's > in place? If that shows a performance problem _then_ it's time to make > this special snowflake just a little more special. Not before. I have looked into the percpu counter code. There are two aspects that I don't like to introduce to the interrupt handler's code path for updating the counts. 1) There is a raw spinlock in the percpu_counter structure that may need to be acquired in the update path. This can be a performance drag especially if lockdep is enabled. 2) The percpu_counter structure is 40 bytes in size on 64-bit systems compared with just 8 bytes for the percpu count pointer and an additional 4 bytes that I introduced in patch 2. With thousands of irq descriptors, it can consume quite a lot more memory. Memory consumption was a point that you brought up in one of your previous mails. Cheers, Longman
