Messages in this thread | | | Date | Fri, 23 Jun 2023 21:10:57 +0800 | Subject | Re: [PATCH 24/29] mm: vmscan: make global slab shrink lockless | From | Qi Zheng <> |
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On 2023/6/23 14:29, Dave Chinner wrote: > On Thu, Jun 22, 2023 at 05:12:02PM +0200, Vlastimil Babka wrote: >> On 6/22/23 10:53, Qi Zheng wrote: >>> @@ -1067,33 +1068,27 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, >>> if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg)) >>> return shrink_slab_memcg(gfp_mask, nid, memcg, priority); >>> >>> - if (!down_read_trylock(&shrinker_rwsem)) >>> - goto out; >>> - >>> - list_for_each_entry(shrinker, &shrinker_list, list) { >>> + rcu_read_lock(); >>> + list_for_each_entry_rcu(shrinker, &shrinker_list, list) { >>> struct shrink_control sc = { >>> .gfp_mask = gfp_mask, >>> .nid = nid, >>> .memcg = memcg, >>> }; >>> >>> + if (!shrinker_try_get(shrinker)) >>> + continue; >>> + rcu_read_unlock(); >> >> I don't think you can do this unlock? >> >>> + >>> ret = do_shrink_slab(&sc, shrinker, priority); >>> if (ret == SHRINK_EMPTY) >>> ret = 0; >>> freed += ret; >>> - /* >>> - * Bail out if someone want to register a new shrinker to >>> - * prevent the registration from being stalled for long periods >>> - * by parallel ongoing shrinking. >>> - */ >>> - if (rwsem_is_contended(&shrinker_rwsem)) { >>> - freed = freed ? : 1; >>> - break; >>> - } >>> - } >>> >>> - up_read(&shrinker_rwsem); >>> -out: >>> + rcu_read_lock(); >> >> That new rcu_read_lock() won't help AFAIK, the whole >> list_for_each_entry_rcu() needs to be under the single rcu_read_lock() to be >> safe. > > Yeah, that's the pattern we've been taught and the one we can look > at and immediately say "this is safe". > > This is a different pattern, as has been explained bi Qi, and I > think it *might* be safe. > > *However.* > > Right now I don't have time to go through a novel RCU list iteration > pattern it one step at to determine the correctness of the > algorithm. I'm mostly worried about list manipulations that can > occur outside rcu_read_lock() section bleeding into the RCU > critical section because rcu_read_lock() by itself is not a memory > barrier. > > Maybe Paul has seen this pattern often enough he could simply tell > us what conditions it is safe in. But for me to work that out from > first principles? I just don't have the time to do that right now.
Hi Paul, can you help to confirm this?
> >> IIUC this is why Dave in [4] suggests unifying shrink_slab() with >> shrink_slab_memcg(), as the latter doesn't iterate the list but uses IDR. > > Yes, I suggested the IDR route because radix tree lookups under RCU > with reference counted objects are a known safe pattern that we can > easily confirm is correct or not. Hence I suggested the unification > + IDR route because it makes the life of reviewers so, so much > easier...
In fact, I originally planned to try the unification + IDR method you suggested at the beginning. But in the case of CONFIG_MEMCG disabled, the struct mem_cgroup is not even defined, and root_mem_cgroup and shrinker_info will not be allocated. This required more code changes, so I ended up keeping the shrinker_list and implementing the above pattern.
If the above pattern is not safe, I will go back to the unification + IDR method.
Thanks, Qi
> > Cheers, > > Dave.
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