On 2023/10/31 00:19, Vlastimil Babka wrote: > On 10/28/23 04:36, Chengming Zhou wrote: >>> >>> >>> After this patch the PG_workingset indicates the state of being on the partial lists. >>> >>> What does "frozen slab" then mean? The slab is being allocated from? Is that information useful or can we drop the frozen flag? >> >> Right, frozen slab is the cpu slab, which is being allocated from by the cpu that froze it. >> >> IMHO, the "frozen" bit is useful because: >> >> 1. PG_workingset is only useful on partial slab, which indicates the slab is on the node >> partial list, so we can manipulate its list in the __slab_free() path. >> >> 2. But for full slab (slab->freelist == NULL), PG_workingset is not much useful, we don't >> safely know whether it's used as the cpu slab or not just from this flag. So __slab_free() >> still rely on the "frozen" bit to know it. > > Well, we could extend the meaning of PG_workingset to mean "not a cpu slab > or pecpu partial slab" i.e. both on node partial list and full. However it > would increase the number of cases where __slab_free() has to lock the > list_lock and check the PG_working set. "slab->freelist == NULL" might > happen often exactly because the freelist became cpu freelist. Ah, right, it's possible to do like this. > >> 3. And the maintaining of "frozen" has no extra cost now, since it's changed together with "freelist" >> and other counter using cmpxchg, we already have the cmpxchg when start to use a slab as the cpu slab. > > And together with this point, I don't see a reason to drop the frozen bit. > It's still useful for cpu slabs. It just wasn't the best possible solution > for percpu partial slabs. > >> Maybe I missed something, I don't know how to drop the frozen flag. > > Should be possible, but not worth it IMHO. Agree, we'll just keep "frozen" for the cpu slabs. Thanks!
