Messages in this thread | | | Date | Wed, 13 Jun 2012 12:21:15 +0800 | From | Fengguang Wu <> | Subject | Re: [PATCH] writeback: avoid race when update bandwidth |
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On Wed, Jun 13, 2012 at 01:56:47PM +1000, Dave Chinner wrote: > On Tue, Jun 12, 2012 at 07:21:29PM +0800, Fengguang Wu wrote: > > On Tue, Jun 12, 2012 at 06:26:43PM +0800, Wanpeng Li wrote: > > > From: Wanpeng Li <liwp@linux.vnet.ibm.com> > > > > That email address is no longer in use? > > > > > Since bdi->wb.list_lock is used to protect the b_* lists, > > > so the flushers who call wb_writeback to writeback pages will > > > stuck when bandwidth update policy holds this lock. In order > > > to avoid this race we can introduce a new bandwidth_lock who > > > is responsible for protecting bandwidth update policy. > > This is not a race condition - it is a lock contention condition.
Nod.
> > This looks good to me. wb.list_lock could be contended and it's better > > for bdi_update_bandwidth() to use a standalone and hardly contended > > lock. > > I'm not sure it will be "hardly contended". That's a global lock, so > now we'll end up with updates on different bdis contending and it's > not uncommon to see a couple of thousand processes on large machines > beating on balance_dirty_pages(). Putting a global scope lock > around such a function doesn't seem like a good solution to me.
It's more about the number of bdi's than the number of processes that matters. Because here is a per-bdi 200ms ratelimit:
bdi_update_bandwidth():
if (time_is_after_eq_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL)) return; // lock it
So a global should be enough when there are only dozens of disks.
However, the global bandwidth_lock will probably become a problem when there comes hundreds of disks. If there are (or will be) such setups, I'm fine to revert to the old per-bdi locking.
> Oh, and if you want to remove the dirty_lock from > global_update_limit(), then replacing the lock with a cmpxchg loop > will do it just fine....
Yes. But to be frank, I don't care about that dirty_lock at all, because it has its own 200ms rate limiting :-)
Thanks, Fengguang
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