[Devel] Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock

[KAMEZAWA Hiroyuki]

On Wed, 14 Apr 2010 10:04:30 -0400
Vivek Goyal <vgoyal at redhat.com> wrote:

> On Wed, Apr 14, 2010 at 06:29:04PM +0900, KAMEZAWA Hiroyuki wrote:

> Hi Kame-san,
> 
> May be I am missing something but how does it solve the issue of making sure
> lock_page_cgroup() is not held in interrupt context? IIUC, above code will
> make sure that for file cache accouting, lock_page_cgroup() is taken only
> if task migration is on. But say task migration is on, and then some IO
> completes and we update WRITEBACK stat (i think this is the one which can
> be called from interrupt context), then we will still take the
> lock_page_cgroup() and again run into the issue of deadlocks?
> 
Yes and No.

At "Set", IIRC, almost all updates against DIRTY and WRITBACK accountings
can be done under mapping->tree_lock, which disables IRQ always.
(ex. I don't mention TestSetPageWriteback but account_page_diritied().)
Then, save/restore irq flags is just a cost and no benefit, in such cases.
Of course, there are cases irqs doesn't enabled.

About FILE_MAPPED, it's not updated under mapping->tree_lock. 
So, we'll have race with charge/uncharge. We have to take lock_page_cgroup(), always.


So, I think we'll have 2 or 3 interfaces, finally.
	mem_cgroup_update_stat_fast()  // the caller must disable IRQ and lock_page()
and
	mem_cgroup_update_stat_locked() // the caller has lock_page().
and
	mem_cgroup_update_stat_safe()  // the caller don't have to do anything.

Why update_stat_fast() is for avoiding _unnecessary_ costs.
When we lock a page and disables IRQ, we don't have to do anything.
There are no races. 

But yes, it's complicated. I'd like to see what we can do.


Thanks,
-Kame


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