Re: [PATCH v3 07/11] mm: vmalloc: Offload free_vmap_area_lock lock

[Dave Chinner]

On Fri, Jan 12, 2024 at 01:18:27PM +0100, Uladzislau Rezki wrote:
> On Fri, Jan 12, 2024 at 07:37:36AM +1100, Dave Chinner wrote:
> > On Thu, Jan 11, 2024 at 04:54:48PM +0100, Uladzislau Rezki wrote:
> > > On Thu, Jan 11, 2024 at 08:02:16PM +1100, Dave Chinner wrote:
> > > > On Tue, Jan 02, 2024 at 07:46:29PM +0100, Uladzislau Rezki (Sony) wrote:
> > > > > Concurrent access to a global vmap space is a bottle-neck.
> > > > > We can simulate a high contention by running a vmalloc test
> > > > > suite.
> > > > > 
> > > > > To address it, introduce an effective vmap node logic. Each
> > > > > node behaves as independent entity. When a node is accessed
> > > > > it serves a request directly(if possible) from its pool.
> > > > > 
> > > > > This model has a size based pool for requests, i.e. pools are
> > > > > serialized and populated based on object size and real demand.
> > > > > A maximum object size that pool can handle is set to 256 pages.
> > > > > 
> > > > > This technique reduces a pressure on the global vmap lock.
> > > > > 
> > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > > > 
> > > > Why not use a llist for this? That gets rid of the need for a
> > > > new pool_lock altogether...
> > > > 
> > > Initially i used the llist. I have changed it because i keep track
> > > of objects per a pool to decay it later. I do not find these locks
> > > as contented one therefore i did not think much.
> > 
> > Ok. I've used llist and an atomic counter to track the list length
> > in the past.
> > 
> > But is the list length even necessary? It seems to me that it is
> > only used by the shrinker to determine how many objects are on the
> > lists for scanning, and I'm not sure that's entirely necessary given
> > the way the current global shrinker works (i.e. completely unfair to
> > low numbered nodes due to scan loop start bias).
> > 
> I use the length to decay pools by certain percentage, currently it is
> 25%, so i need to know number of objects. It is done in the purge path.
> As for shrinker, once it hits us we drain pools entirely.

Why does purge need to be different to shrinking?

But, regardless, you can still use llist with an atomic counter to
do this - there is no need for a spin lock at all.

> > > Anyway, i will have a look at this to see if llist is easy to go with
> > > or not. If so i will send out a separate patch.
> > 
> > Sounds good, it was just something that crossed my mind given the
> > pattern of "producer adds single items, consumer detaches entire
> > list, processes it and reattaches remainder" is a perfect match for
> > the llist structure.
> > 
> The llist_del_first() has to be serialized. For this purpose a per-cpu
> pool would work or kind of "in_use" atomic that protects concurrent
> removing.

So don't use llist_del_first().

> If we detach entire llist, then we need to keep track of last node
> to add it later as a "batch" to already existing/populated list.

Why? I haven't see any need for ordering these lists which would
requiring strict tail-add ordered semantics.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx