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Date: Mon, 13 Jan 2020 11:24:37 -0500 From: Waiman Long <longman@...hat.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org> Subject: Re: [PATCH v2 4/6] locking/lockdep: Reuse freed chain_hlocks entries On 1/13/20 10:58 AM, Peter Zijlstra wrote: > On Mon, Dec 16, 2019 at 10:15:15AM -0500, Waiman Long wrote: >> +/* >> + * Return offset of a chain block of the right size or -1 if not found. >> + */ >> +static inline int alloc_chain_hlocks_from_buckets(int size) >> +{ >> + int prev, curr, next; >> + >> + if (!nr_free_chain_hlocks) >> + return -1; >> + >> + if (size <= MAX_CHAIN_BUCKETS) { >> + curr = chain_block_buckets[size - 1]; >> + if (curr < 0) >> + return -1; >> + >> + chain_block_buckets[size - 1] = next_chain_block(curr); >> + nr_free_chain_hlocks -= size; >> + return curr; >> + } >> + >> + /* >> + * Look for a free chain block of the given size >> + * >> + * It is rare to have a lock chain with depth > MAX_CHAIN_BUCKETS. >> + * It is also more expensive as we may iterate the whole list >> + * without finding one. >> + */ >> + prev = -1; >> + curr = chain_block_buckets[0]; >> + while (curr >= 0) { >> + next = next_chain_block(curr); >> + if (chain_block_size(curr) == size) { >> + set_chain_block(prev, 0, next); >> + nr_free_chain_hlocks -= size; >> + nr_large_chain_blocks--; >> + return curr; >> + } >> + prev = curr; >> + curr = next; >> + } >> + return -1; >> +} >> +/* >> + * The graph lock must be held before calling this function. >> + * >> + * Return: an offset to chain_hlocks if successful, or >> + * -1 with graph lock released >> + */ >> +static int alloc_chain_hlocks(int size) >> +{ >> + int curr; >> + >> + if (size < 2) >> + size = 2; >> + >> + curr = alloc_chain_hlocks_from_buckets(size); >> + if (curr >= 0) >> + return curr; >> + >> + BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks)); >> + BUILD_BUG_ON((1UL << 6) <= ARRAY_SIZE(current->held_locks)); >> + BUILD_BUG_ON((1UL << 8*sizeof(chain_hlocks[0])) <= >> + ARRAY_SIZE(lock_classes)); >> + >> + /* >> + * Allocate directly from chain_hlocks. >> + */ >> + if (likely(nr_chain_hlocks + size <= MAX_LOCKDEP_CHAIN_HLOCKS)) { >> + curr = nr_chain_hlocks; >> + nr_chain_hlocks += size; >> + return curr; >> + } >> + if (!debug_locks_off_graph_unlock()) >> + return -1; >> + >> + print_lockdep_off("BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!"); >> + dump_stack(); >> + return -1; >> +} > *groan*.... > > That's _two_ allocators :/ And it can trivially fail, even if there's > plenty space available. > > Consider nr_chain_hlocks is exhaused, and @size is empty, but size+1 > still has blocks. > > I'm guessing you didn't make it a single allocator because you didn't > want to implement block splitting? why? > In my testing, most of the lock chains tend to be rather short (within the 2-8 range). I don't see a lot of free blocks left in the system after the test. So I don't see a need to implement block splitting for now. If you think this is a feature that needs to be implemented for the patch to be complete, I can certainly add patch to do that. My initial thought is just to split long blocks in the unsized list for allocation request that is no longer than 8 to make thing easier. Cheers, Longman
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