On 14/11/2024 09:05, Christian König wrote:
Am 13.11.24 um 18:19 schrieb Tvrtko Ursulin:From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx> One alternative to the fix Christian proposed in https://lore.kernel.org/dri-devel/20241024124159.4519-3-christian.koenig@xxxxxxx/ is to replace the rather complex open coded sorting loops with the kernel standard sort followed by a context squashing pass. Proposed advantage of this would be readability but one concern Christianraised was that there could be many fences, that they are typically mostlysorted, and so the kernel's heap sort would be much worse by the proposed algorithm. I had a look running some games and vkcube to see what are the typical number of input fences. Tested scenarios: 1) Hogwarts Legacy under Gamescope 450 calls per second to __dma_fence_unwrap_merge. Percentages per number of fences buckets, before and after checking for signalled status, sorting and flattening: N Before After 0 0.91% 1 69.40% 2-3 28.72% 9.4% (90.6% resolved to one fence) 4-5 0.93% 6-9 0.03% 10+ 2) Cyberpunk 2077 under Gamescope 1050 calls per second, amounting to 0.01% CPU time according to perf top. N Before After 0 1.13% 1 52.30% 2-3 40.34% 55.57% 4-5 1.46% 0.50% 6-9 2.44% 10+ 2.34% 3) vkcube under Plasma 90 calls per second. N Before After 0 1 2-3 100% 0% (Ie. all resolved to a single fence) 4-5 6-9 10+ In the case of vkcube all invocations in the 2-3 bucket were actually just two input fences. From these numbers it looks like the heap sort should not be a disadvantage, given how the dominant case is <= 2 input fences which heapsort solves with just one compare and swap. (And for the case of one inputfence we have a fast path in the previous patch.) A complementary possibility is to implement a different sorting algorithm under the same API as the kernel's sort() and so keep the simplicity, potentially moving the new sort under lib/ if it would be found more widely useful.Well the API would need to be different from sort() since a merge sort always works with multiple inputs and a single output.
I was thinking insert sort could be good for small arrays if they are mostly already sorted. Reference I found was https://dl.acm.org/doi/pdf/10.1145/359024.359026, and although they do not look at lists below 50 elements, I think advantage over heap would hold even better.
But from the number you gathered I really don't think we are going to need that.Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx> Cc: Christian König <christian.koenig@xxxxxxx> Cc: Friedrich Vock <friedrich.vock@xxxxxx> --- drivers/dma-buf/dma-fence-unwrap.c | 129 ++++++++++++++++------------- 1 file changed, 73 insertions(+), 56 deletions(-)diff --git a/drivers/dma-buf/dma-fence-unwrap.c b/drivers/dma-buf/dma-fence-unwrap.cindex 75c3e37fd617..750dc20a9e9d 100644 --- a/drivers/dma-buf/dma-fence-unwrap.c +++ b/drivers/dma-buf/dma-fence-unwrap.c @@ -12,6 +12,7 @@ #include <linux/dma-fence-chain.h> #include <linux/dma-fence-unwrap.h> #include <linux/slab.h> +#include <linux/sort.h> /* Internal helper to start new array iteration, don't use directly */ static struct dma_fence *@@ -59,6 +60,25 @@ struct dma_fence *dma_fence_unwrap_next(struct dma_fence_unwrap *cursor)} EXPORT_SYMBOL_GPL(dma_fence_unwrap_next); + +static int fence_cmp(const void *_a, const void *_b) +{ + struct dma_fence *a = *(struct dma_fence **)_a; + struct dma_fence *b = *(struct dma_fence **)_b; + + if (a->context < b->context) + return -1; + else if (a->context > b->context) + return 1; + + if (dma_fence_is_later(b, a)) + return -1; + else if (dma_fence_is_later(a, b)) + return 1; + + return 0; +} +/* Implementation for the dma_fence_merge() marco, don't use directly */struct dma_fence *__dma_fence_unwrap_merge(unsigned int num_fences, struct dma_fence **fences,@@ -67,9 +87,12 @@ struct dma_fence *__dma_fence_unwrap_merge(unsigned int num_fences,struct dma_fence *tmp, *signaled, **array; struct dma_fence_array *result; ktime_t timestamp; - unsigned int i; - size_t count; + int i, j, count; + /* + * Count number of unwrapped fences and fince the latest signaled + * timestamp. + */What is done should be obvious from the code, only why something is done needs code comment and explanation.
I was going for completing the narrative so each logical block in the function has a comment, versus just some. IMO it makes it easier to follow by making the steps nicely visually separated. But I don't feel strongly about this so have removed it in v2.
count = 0; timestamp = ns_to_ktime(0); for (i = 0; i < num_fences; ++i) {@@ -98,74 +121,68 @@ struct dma_fence *__dma_fence_unwrap_merge(unsigned int num_fences,else if (count == 1) return dma_fence_get(signaled); + /* + * Allocate and populate the array. + */ array = kmalloc_array(count, sizeof(*array), GFP_KERNEL); if (!array) return NULL; - /*- * This trashes the input fence array and uses it as position for the- * following merge loop. This works because the dma_fence_merge()- * wrapper macro is creating this temporary array on the stack together- * with the iterators. - */ - for (i = 0; i < num_fences; ++i) - fences[i] = dma_fence_unwrap_first(fences[i], &iter[i]); - count = 0; - do { - unsigned int sel; - -restart: - tmp = NULL; - for (i = 0; i < num_fences; ++i) { - struct dma_fence *next; - - while (fences[i] && dma_fence_is_signaled(fences[i])) - fences[i] = dma_fence_unwrap_next(&iter[i]); - - next = fences[i]; - if (!next) - continue; - - /* - * We can't guarantee that inpute fences are ordered by - * context, but it is still quite likely when this - * function is used multiple times. So attempt to order - * the fences by context as we pass over them and merge - * fences with the same context. - */ - if (!tmp || tmp->context > next->context) { - tmp = next; - sel = i; - - } else if (tmp->context < next->context) { - continue; - - } else if (dma_fence_is_later(tmp, next)) { - fences[i] = dma_fence_unwrap_next(&iter[i]); - goto restart; - } else { - fences[sel] = dma_fence_unwrap_next(&iter[sel]); - goto restart; - } + for (i = 0; i < num_fences; ++i) { + dma_fence_unwrap_for_each(tmp, &iter[i], fences[i]) { + if (!dma_fence_is_signaled(tmp)) + array[count++] = tmp;Same problem as in patch #1, you need to grab a reference to tmp here.
Yep. v2 will appear on the list shortly and given my sloppy quality track record when jumping between the topics I would appreciate if someone else would smoke test it too.
Regards, Tvrtko
Apart from that the patch looks good to me, but I would reduce the comments.When we need to explain what code does then the code need to be improved and not documented.Regards, Christian} - - if (tmp) { - array[count++] = dma_fence_get(tmp); - fences[sel] = dma_fence_unwrap_next(&iter[sel]); + } + + /*+ * Equal fast-path as the above one, in case some fences got signalled+ * in the meantime. + */ + if (count == 0) { + tmp = dma_fence_allocate_private_stub(timestamp); + goto return_tmp; + } else if (count == 1) { + tmp = dma_fence_get(array[0]); + goto return_tmp; + } + + /* + * Sort in context and seqno order. + */ + sort(array, count, sizeof(*array), fence_cmp, NULL); + + /* + * Only keep the most recent fence for each context. + */ + j = 0; + tmp = array[0]; + for (i = 1; i < count; i++) { + if (array[i]->context != tmp->context) { + array[j++] = dma_fence_get(tmp); } - } while (tmp); - + tmp = array[i]; + } + if (j == 0 || tmp->context != array[j - 1]->context) { + array[j++] = dma_fence_get(tmp); + } + count = j; + + /* + * And another fast-path as the earlier ones. + */ if (count == 0) { tmp = dma_fence_allocate_private_stub(ktime_get()); goto return_tmp; - } - - if (count == 1) { + } else if (count == 1) { tmp = array[0]; goto return_tmp; } + /* + * Finnaly create the output fence array. + */ result = dma_fence_array_create(count, array, dma_fence_context_alloc(1), 1, false);
