That is one aspect. When performing writeback then we need to figure out
which inodes have dirty pages in the memcg and we need to start writeout
on those inodes and not on others that have their dirty pages elsewhere.
There are two components of this that are in this patch and that would
also have to be implemented for a memcg.
Doable. lru->page->mapping->host is a good start.
This patch would solve the problem if the calculation of the dirty pages
would consider the active memcg and be able to determine the amount of
dirty pages (through some sort of additional memcg counters). That is just
the first part though. The second part of finding the inodes that have
dirty pages for writeback would require an association between memcgs and
inodes.
We presently have that via the LRU. It has holes, but so does this per-cpuset
scheme.
Generally, I worry that this is a specific fix to a specific problem
encountered on specific machines with specific setups and specific
workloads, and that it's just all too low-level and myopic.
And now we're back in the usual position where there's existing code and
everyone says it's terribly wonderful and everyone is reluctant to step
back and look at the big picture. Am I wrong?
Plus: we need per-memcg dirty-memory throttling, and this is more
important than per-cpuset, I suspect. How will the (already rather
buggy) code look once we've stuffed both of them in there?
I agree that there's a problem here, although given the amount of time
that it's been there, I suspect that it is a very small problem.
Someone please convince me that in three years time we will agree that
merging this fix to that problem was a correct decision?