Hi Dennis, > -----Original Message----- > From: Dennis Zhou [mailto:dennis@xxxxxxxxxx] > Sent: 2019年2月27日 1:33 > To: Peng Fan <peng.fan@xxxxxxx> > Cc: dennis@xxxxxxxxxx; tj@xxxxxxxxxx; cl@xxxxxxxxx; linux-mm@xxxxxxxxx; > linux-kernel@xxxxxxxxxxxxxxx; van.freenix@xxxxxxxxx > Subject: Re: [RFC] percpu: decrease pcpu_nr_slots by 1 > > On Tue, Feb 26, 2019 at 12:09:28AM +0000, Peng Fan wrote: > > Hi Dennis, > > > > > -----Original Message----- > > > From: dennis@xxxxxxxxxx [mailto:dennis@xxxxxxxxxx] > > > Sent: 2019年2月25日 23:24 > > > To: Peng Fan <peng.fan@xxxxxxx> > > > Cc: tj@xxxxxxxxxx; cl@xxxxxxxxx; linux-mm@xxxxxxxxx; > > > linux-kernel@xxxxxxxxxxxxxxx; van.freenix@xxxxxxxxx > > > Subject: Re: [RFC] percpu: decrease pcpu_nr_slots by 1 > > > > > > On Sun, Feb 24, 2019 at 09:17:08AM +0000, Peng Fan wrote: > > > > Entry pcpu_slot[pcpu_nr_slots - 2] is wasted with current code logic. > > > > pcpu_nr_slots is calculated with `__pcpu_size_to_slot(size) + 2`. > > > > Take pcpu_unit_size as 1024 for example, __pcpu_size_to_slot will > > > > return max(11 - PCPU_SLOT_BASE_SHIFT + 2, 1), it is 8, so the > > > > pcpu_nr_slots will be 10. > > > > > > > > The chunk with free_bytes 1024 will be linked into pcpu_slot[9]. > > > > However free_bytes in range [512,1024) will be linked into > > > > pcpu_slot[7], because `fls(512) - PCPU_SLOT_BASE_SHIFT + 2` is 7. > > > > So pcpu_slot[8] is has no chance to be used. > > > > > > > > According comments of PCPU_SLOT_BASE_SHIFT, 1~31 bytes share the > > > same > > > > slot and PCPU_SLOT_BASE_SHIFT is defined as 5. But actually 1~15 > > > > share the same slot 1 if we not take PCPU_MIN_ALLOC_SIZE into > > > > consideration, > > > > 16~31 share slot 2. Calculation as below: > > > > highbit = fls(16) -> highbit = 5 > > > > max(5 - PCPU_SLOT_BASE_SHIFT + 2, 1) equals 2, not 1. > > > > > > > > This patch by decreasing pcpu_nr_slots to avoid waste one slot and > > > > let [PCPU_MIN_ALLOC_SIZE, 31) really share the same slot. > > > > > > > > Signed-off-by: Peng Fan <peng.fan@xxxxxxx> > > > > --- > > > > > > > > V1: > > > > Not very sure about whether it is intended to leave the slot there. > > > > > > > > mm/percpu.c | 4 ++-- > > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/mm/percpu.c b/mm/percpu.c index > > > > 8d9933db6162..12a9ba38f0b5 100644 > > > > --- a/mm/percpu.c > > > > +++ b/mm/percpu.c > > > > @@ -219,7 +219,7 @@ static bool pcpu_addr_in_chunk(struct > > > > pcpu_chunk *chunk, void *addr) static int __pcpu_size_to_slot(int size) > { > > > > int highbit = fls(size); /* size is in bytes */ > > > > - return max(highbit - PCPU_SLOT_BASE_SHIFT + 2, 1); > > > > + return max(highbit - PCPU_SLOT_BASE_SHIFT + 1, 1); > > > > } > > > > > > Honestly, it may be better to just have [1-16) [16-31) be separate. Missed to reply this in previous thread, the following comments let me think the chunk slot calculation might be wrong, so this comment needs to be updated, saying "[PCPU_MIN_ALLOC_SIZE - 15) bytes share the same slot", if [1-16)[16-31) is expected. " /* the slots are sorted by free bytes left, 1-31 bytes share the same slot */ #define PCPU_SLOT_BASE_SHIFT 5 " > > > I'm working on a change to this area, so I may change what's going on > here. > > > > > > > > > > > static int pcpu_size_to_slot(int size) @@ -2145,7 +2145,7 @@ int > > > > __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai, > > > > * Allocate chunk slots. The additional last slot is for > > > > * empty chunks. > > > > */ > > > > - pcpu_nr_slots = __pcpu_size_to_slot(pcpu_unit_size) + 2; > > > > + pcpu_nr_slots = __pcpu_size_to_slot(pcpu_unit_size) + 1; > > > > pcpu_slot = memblock_alloc(pcpu_nr_slots * sizeof(pcpu_slot[0]), > > > > SMP_CACHE_BYTES); > > > > for (i = 0; i < pcpu_nr_slots; i++) > > > > -- > > > > 2.16.4 > > > > > > > > > > This is a tricky change. The nice thing about keeping the additional > > > slot around is that it ensures a distinction between a completely > > > empty chunk and a nearly empty chunk. > > > > Are there any issues met before if not keeping the unused slot? > > From reading the code and git history I could not find information. > > I tried this code on aarch64 qemu and did not meet issues. > > > > This change would require verification that all paths lead to power of 2 chunk > sizes and most likely a BUG_ON if that's not the case. I try to understand, "power of 2 chunk sizes", you mean the runtime free_bytes of a chunk? > > So while this would work, we're holding onto an additional slot also to be used > for chunk reclamation via pcpu_balance_workfn(). If a chunk was not a power > of 2 resulting in the last slot being entirely empty chunks we could free stuff a > chunk with addresses still in use. You mean the following code might free stuff when a percpu variable is still being used if the chunk runtime free_bytes is not a power of 2? " 1623 list_for_each_entry_safe(chunk, next, &to_free, list) { 1624 int rs, re; 1625 1626 pcpu_for_each_pop_region(chunk->populated, rs, re, 0, 1627 chunk->nr_pages) { 1628 pcpu_depopulate_chunk(chunk, rs, re); 1629 spin_lock_irq(&pcpu_lock); 1630 pcpu_chunk_depopulated(chunk, rs, re); 1631 spin_unlock_irq(&pcpu_lock); 1632 } 1633 pcpu_destroy_chunk(chunk); 1634 cond_resched(); 1635 } " > > > > It happens to be that the logic creates power of 2 chunks which ends > > > up being an additional slot anyway. > > > > > > So, > > > given that this logic is tricky and architecture dependent, > > > > Could you share more information about architecture dependent? > > > > The crux of the logic is in pcpu_build_alloc_info(). It's been some time since > I've thought deeply about it, but I don't believe there is a guarantee that it will > be a power of 2 chunk. I am a bit lost about a power of 2, need to read more about the code. Thanks, Peng. > > Thanks, > Dennis
