Messages in this thread | | | From | Rusty Russell <> | Subject | Re: per-cpu stats in block device: overkill? | Date | Mon, 22 Dec 2008 14:26:09 +1030 |
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On Monday 22 December 2008 12:25:54 Tejun Heo wrote: > I'm working on local counter (local_t) allocator which is used to > replace percpu allocation in percpu_counter and used as basis for > percpu_ref which replaces module ref counting and will be used to > simplify block/char lifetime rules.
Hi Tejun,
Interesting! Thanks to Christoph's dynamic percpu efforts, I've been revising efforts to make alloc_percpu use the same efficient mechanism that static percpu vars use. We actually have this code already, tucked away in module.c.
This work is basically complete; the step I started this morning is to remove the per_cpu__ prefix hackery from the per-cpu ops (in favour of sparse annotations). This leads to cpu_local_inc et. al. being usable for alloc_percpu-created percpu vars, not just static ones.
> The local counter allocator allocates per-cpu pages and the space > overhead is minimal. If per-cpu stats in genhd is necessary, I think > converting it to percpu local counter allocation should do it.
Interesting; an allyesconfig boot uses 194 per-cpu allocs from lib/percpu_counter.c at the moment. The module.c allocator is fairly space efficient: 4 bytes per "block" (ie. each allocation or hole) but slow, which I figure is OK. Packing is good though.
> BTW, why make percpu area static?
Good question. Archs use a simple offset for per-cpu areas: some hold this in a register (eg. %fs for x86-32). This means that the layout must be "congruent" (ie. have the same inter-cpu spacing) if we allocate a new per-cpu area (hard for non-NUMA).
For 5 years I waited for this to be fixed, and avoided exposing the per-cpu core, and the alloc_percpu stuff was a standin implementation. But Christoph L. showed that even with the size limit, there are numerous places which want small per-cpu allocations which are optimally accessed, so I restarted work. See Message-Id: <20081117132630.33F09DDDF5@ozlabs.org> "[PATCH 1/7] Improve alloc_percpu: make the per cpu reserve configurable and larger." and thread.
In addition, Mathieu and I have been discussing local_t: it's wandered off its original purpose and we're debating what to do about it. See Message-ID: <alpine.DEB.1.10.0812150823370.18692@gandalf.stny.rr.com> "local_add_return" and thread.
I look forward to always-cogent your thoughts on these issues! Rusty.
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