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. 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. 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. > > 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. Thanks, Dennis
