Messages in this thread | | | Date | Sat, 22 Nov 2008 10:48:07 +0100 | From | Joerg Roedel <> | Subject | Re: [PATCH 03/10] x86: add initialization code for DMA-API debugging |
| |
On Fri, Nov 21, 2008 at 06:43:48PM +0100, Ingo Molnar wrote: > > * Joerg Roedel <joerg.roedel@amd.com> wrote: > > > +static struct list_head dma_entry_hash[HASH_SIZE]; > > + > > +/* A slab cache to allocate dma_map_entries fast */ > > +static struct kmem_cache *dma_entry_cache; > > + > > +/* lock to protect the data structures */ > > +static DEFINE_SPINLOCK(dma_lock); > > some more generic comments about the data structure: it's main purpose > is to provide a mapping based on (dev,addr). There's little if any > cross-entry interaction - same-address+same-dev DMA is checked. > > 1) > > the hash: > > + return (entry->dev_addr >> HASH_FN_SHIFT) & HASH_FN_MASK; > > should mix in entry->dev as well - that way we get not just per > address but per device hash space separation as well. > > 2) > > HASH_FN_SHIFT is 1MB chunks right now - that's probably fine in > practice albeit perhaps a bit too small. There's seldom any coherency > between the physical addresses of DMA - we rarely have any real > (performance-relevant) physical co-location of DMA addresses beyond 4K > granularity. So using 1MB chunking here will discard a good deal of > random low bits we should be hashing on. > > 3) > > And the most scalable locking would be per hash bucket locking - no > global lock is needed. The bucket hash heads should probably be > cacheline sized - so we'd get one lock per bucket.
Hmm, I just had the idea of saving this data in struct device. How about that? The locking should scale too and we can extend it easier. For example it simplifys a per-device disable function for the checking. Or another future feature might be leak tracing.
> This way if there's irq+DMA traffic on one CPU from one device into > one range of memory, and irq+DMA traffic on another CPU to another > device, they will map to two different hash buckets. > > 4) > > Plus it might be an option to make hash lookup lockless as well: > depending on the DMA flux we can get a lot of lookups, and taking the > bucket lock can be avoided, if you use RCU-safe list ops and drive the > refilling of the free entries pool from RCU.
Joerg
| |