Re: [MM] Make mm counters per cpu instead of atomic V2

From: Christoph Lameter
Date: Fri Nov 06 2009 - 14:15:19 EST

On Sat, 7 Nov 2009, KAMEZAWA Hiroyuki wrote:

> And allocate mm->usage only when the first CLONE_THREAD is specified.


> if (mm->usage)
> access per cpu
> else
> atomic_long_xxx

If we just have one thread: Do we need atomic access at all?

> and read operation will be
> val = atomic_read(mm->rss);
> if (mm->usage)
> for_each_possible_cpu()....

val = m->rss
for_each_cpu(cpu) val+= percpu ...

> ==
> Does "if" seems too costly ?

The above method would avoid the if.

> If this idea is bad, I think moving mm_counter to task_struct from
> mm_struct and doing slow-sync is an idea instead of percpu.

Yeah then the access is effectively percpu as long as preempt is disabled.

But then for the mmap_writer_lock we would need to traverse a doubly
linked list to add up the counters. Bad caching on that one and we would
have to lock the list too. Sigh.

> kswapd and reclaim routine can update mm_struct's counter, directly.
> Readers just read mm_struct's counter.

Would work for rss counters but not for avoiding the rw semaphore I guess.

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