Messages in this thread | ![/](/images/icornerl.gif) | | Date | Sat, 24 Jun 2023 19:08:18 +0800 | Subject | Re: [PATCH 24/29] mm: vmscan: make global slab shrink lockless | From | Qi Zheng <> |
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Hi Dave,
On 2023/6/24 06:19, Dave Chinner wrote: > On Fri, Jun 23, 2023 at 09:10:57PM +0800, Qi Zheng wrote: >> 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: >>> 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. > > Yes. Go back and read what I originally said needed to be done > first. In the case of CONFIG_MEMCG=n, a dummy root memcg still needs > to exist that holds all of the global shrinkers. Then shrink_slab() > is only ever passed a memcg that should be iterated. > > Yes, it needs changes external to the shrinker code itself to be > made to work. And even if memcg's are not enabled, we can still use > the memcg structures to ensure a common abstraction is used for the > shrinker tracking infrastructure....
Yeah, what I imagined before was to define a more concise struct mem_cgroup in the case of CONFIG_MEMCG=n, then allocate a dummy root memcg on system boot:
#ifdef !CONFIG_MEMCG
struct shrinker_info { struct rcu_head rcu; atomic_long_t *nr_deferred; unsigned long *map; int map_nr_max; };
struct mem_cgroup_per_node { struct shrinker_info __rcu *shrinker_info; };
struct mem_cgroup { struct mem_cgroup_per_node *nodeinfo[]; };
#endif
But I have a concern: if all global shrinkers are tracking with the info->map of root memcg, a shrinker->id needs to be assigned to them, which will cause info->map_nr_max to become larger than before, then making the traversal of info->map slower.
> >> If the above pattern is not safe, I will go back to the unification + >> IDR method. > > And that is exactly how we got into this mess in the first place....
I only found one similar pattern in the kernel:
fs/smb/server/oplock.c:find_same_lease_key/smb_break_all_levII_oplock/lookup_lease_in_table
But IIUC, the refcount here needs to be decremented after holding rcu lock as I did above.
So regardless of whether we choose unification + IDR in the end, I still want to confirm whether the pattern I implemented above is safe. :)
Thanks, Qi
> > -Dave
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