"We don't want to introduce pointless delays in throttle_vm_writeout() when the writeback limits are not yet exceeded, do we?" asked Fengguang Wu as the description of his patch to
mm/page-writeback.c. Andrew Morton replied, "this is a pretty major bugfix, explaining, "this patch has the potential to significantly alter the dynamics of the VM behaviour under particular workloads. It might turn up other stuff..." He continued:
"I wonder why nobody noticed this happening. Either a) it turns out that kswapd is doing a good job and such callers don't do direct reclaim much or b) nobody is doing any in-depth kernel instrumentation.
"Now, how _would_ one notice this problem? We don't have very good tools, really. Booting with "profile=sleep" and looking at the profile data would be one way. Repeatedly doing sysrq-T is another. Perhaps the new lockstat-via-lockdep code would allow this to be observed in some fashion, dunno."