Messages in this thread | | | Subject | Re: [PATCH 4/5] locking/lockdep: Reuse free chain_hlocks entries | From | Waiman Long <> | Date | Fri, 13 Dec 2019 11:02:46 -0500 |
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On 12/13/19 5:50 AM, Peter Zijlstra wrote: > On Fri, Dec 13, 2019 at 11:25:25AM +0100, Peter Zijlstra wrote: > >> void push_block(struct chain_block **bp, struct chain_block *b) >> { >> b->next = *bp; >> *bp = b; >> } >> >> /* could contemplate ilog2() buckets */ >> int size2bucket(int size) >> { >> return size >= MAX_BUCKET ? 0 : size; >> } > If you make the allocation granularity 4 entries, then you can have > push_block() store the chain_block * at the start of every free entry, > which would enable merging adjecent blocks. > > That is, if I'm not mistaken these u16 chain entries encode the class > index (per lock_chain_get_class()). And since the lock_classes array is > MAX_LOCKDEP_KEYS, or 13 bits, big, we have the MSB of each entry spare. > > union { > struct { > u16 hlock[4]; > } > u64 addr; > } ponies; > > So if we make the rule that: > > !(idx % 4) && (((union ponies *)chain_hlock[idx])->addr & BIT_ULL(63)) > > encodes a free block at said addr, then we should be able to detect if: > > chain_hlock[b->base + b->size] > > is also free and merge the blocks. > > (we just have to fix up the MSB of the address, not all arches have > negative addresses for kernel space) > That is an interesting idea. It will eliminate the need of a separate array to track the free chain_hlocks. However, if there are n chains available, it will waste about 3n bytes of storage, on average.
I have a slightly different idea. I will enforce a minimum allocation size of 2. For a free block, the first 2 hlocks for each allocation block will store a 32-bit integer (hlock[0] << 16)|hlock[1]:
Bit 31: always 1 Bits 24-30: block size Bits 0-23: index to the next free block.
In this way, the wasted space will be k bytes where k is the number of 1-entry chains. I don't think merging adjacent blocks will be that useful at this point. We can always add this capability later on if it is found to be useful.
Cheers, Longman
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