Messages in this thread | | | Date | Fri, 13 Dec 2019 19:12:55 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/5] locking/lockdep: Reuse free chain_hlocks entries |
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On Fri, Dec 13, 2019 at 11:02:46AM -0500, Waiman Long wrote:
> 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.
If you look closely at the proposed allocator, my blocks can be much larger than 7 bit. In fact, it start with a single block of MAX_LOCKDEP_CHAIN_HLOCKS entries.
That said; I don't think you need to encode the size at all. All we need to do is encode the chain_blocks[] index (and stick init_block in that array). That should maybe even fit in a single u16.
Also, if we store that in the first and last 'word' of the free range, we can detect both before and after freespace.
> 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.
I'm thinking 1 entry isn't much of a chain. My brain is completely fried atm, but are we really storing single entry 'chains' ? It seems to me we could skip that.
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