On 28/04/2023 23.38, Toke Høiland-Jørgensen wrote:
Jesper Dangaard Brouer <brouer@xxxxxxxxxx> writes:This removes the workqueue scheme that periodically tests when inflight reach zero such that page_pool memory can be freed. This change adds code to fast-path free checking for a shutdown flags bit after returning PP pages. Performance is very important for PP, as the fast path is used for XDP_DROP use-cases where NIC drivers recycle PP pages directly into PP alloc cache. This patch (since V3) shows zero impact on this fast path. Micro benchmarked with [1] on Intel CPU E5-1650 @3.60GHz. The slight code reorg of likely() are deliberate.Oh, you managed to get rid of the small difference you were seeing before? Nice! :)
Yes! - fast path is not affected at all!!! :-) This is due to this small change in the V3. + if (page && pool->p.flags & PP_FLAG_SHUTDOWN) + page_pool_empty_ring(pool); Due to V3 change of moving free_attempt to page_pool_release_page() the page_pool_empty_ring() call (in page_pool_put_defragged_page) should only be called when (page != NULL). This moves the ASM code for the PP_FLAG_SHUTDOWN out of the fast-path. I also measure the slightly slower path, recycle into ptr_ring, where I cannot really see any effect. It is easier to understand, why/when is it important, when looking at the percentage effect of the measured 0.3 nanosec overhead. For fast-path this would be approx 7%, while for this recycle path it is approx 1.8%.
Just a few questions, see below:[1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/bench_page_pool_simple.c Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx> --- include/net/page_pool.h | 9 +-- net/core/page_pool.c | 138 ++++++++++++++++++++++++++++++++++------------- 2 files changed, 103 insertions(+), 44 deletions(-) diff --git a/include/net/page_pool.h b/include/net/page_pool.h index c8ec2f34722b..a71c0f2695b0 100644 --- a/include/net/page_pool.h +++ b/include/net/page_pool.h @@ -50,6 +50,9 @@ PP_FLAG_DMA_SYNC_DEV |\ PP_FLAG_PAGE_FRAG)+/* Internal flag: PP in shutdown phase, waiting for inflight pages */+#define PP_FLAG_SHUTDOWN BIT(8) + /* * Fast allocation side cache array/stack * @@ -151,11 +154,6 @@ static inline u64 *page_pool_ethtool_stats_get(u64 *data, void *stats) struct page_pool { struct page_pool_params p;- struct delayed_work release_dw;- void (*disconnect)(void *); - unsigned long defer_start; - unsigned long defer_warn; - u32 pages_state_hold_cnt; unsigned int frag_offset; struct page *frag_page; @@ -165,6 +163,7 @@ struct page_pool { /* these stats are incremented while in softirq context */ struct page_pool_alloc_stats alloc_stats; #endif + void (*disconnect)(void *); u32 xdp_mem_id;/*diff --git a/net/core/page_pool.c b/net/core/page_pool.c index e212e9d7edcb..54bdd140b7bd 100644 --- a/net/core/page_pool.c +++ b/net/core/page_pool.c @@ -23,9 +23,6 @@#include <trace/events/page_pool.h> -#define DEFER_TIME (msecs_to_jiffies(1000))-#define DEFER_WARN_INTERVAL (60 * HZ) - #define BIAS_MAX LONG_MAX#ifdef CONFIG_PAGE_POOL_STATS@@ -380,6 +377,10 @@ static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool, struct page *page; int i, nr_pages;+ /* API usage BUG: PP in shutdown phase, cannot alloc new pages */+ if (WARN_ON(pool->p.flags & PP_FLAG_SHUTDOWN)) + return NULL; + /* Don't support bulk alloc for high-order pages */ if (unlikely(pp_order)) return __page_pool_alloc_page_order(pool, gfp); @@ -450,10 +451,9 @@ EXPORT_SYMBOL(page_pool_alloc_pages); */ #define _distance(a, b) (s32)((a) - (b))-static s32 page_pool_inflight(struct page_pool *pool)+static s32 __page_pool_inflight(struct page_pool *pool, + u32 hold_cnt, u32 release_cnt) { - u32 release_cnt = atomic_read(&pool->pages_state_release_cnt); - u32 hold_cnt = READ_ONCE(pool->pages_state_hold_cnt); s32 inflight;inflight = _distance(hold_cnt, release_cnt);@@ -464,6 +464,17 @@ static s32 page_pool_inflight(struct page_pool *pool) return inflight; }+static s32 page_pool_inflight(struct page_pool *pool)+{ + u32 hold_cnt = READ_ONCE(pool->pages_state_hold_cnt); + u32 release_cnt = atomic_read(&pool->pages_state_release_cnt); + return __page_pool_inflight(pool, hold_cnt, release_cnt); +} + +static int page_pool_free_attempt(struct page_pool *pool, + u32 hold_cnt, u32 release_cnt); +static u32 pp_read_hold_cnt(struct page_pool *pool); + /* Disconnects a page (from a page_pool). API users can have a need * to disconnect a page (from a page_pool), to allow it to be used as * a regular page (that will eventually be returned to the normal @@ -471,8 +482,10 @@ static s32 page_pool_inflight(struct page_pool *pool) */ void page_pool_release_page(struct page_pool *pool, struct page *page) { + unsigned int flags = READ_ONCE(pool->p.flags); dma_addr_t dma; - int count; + u32 release_cnt; + u32 hold_cnt;if (!(pool->p.flags & PP_FLAG_DMA_MAP))/* Always account for inflight pages, even if we didn't @@ -490,11 +503,15 @@ void page_pool_release_page(struct page_pool *pool, struct page *page) skip_dma_unmap: page_pool_clear_pp_info(page);- /* This may be the last page returned, releasing the pool, so- * it is not safe to reference pool afterwards. - */ - count = atomic_inc_return_relaxed(&pool->pages_state_release_cnt); - trace_page_pool_state_release(pool, page, count); + if (flags & PP_FLAG_SHUTDOWN) + hold_cnt = pp_read_hold_cnt(pool); + + release_cnt = atomic_inc_return(&pool->pages_state_release_cnt); + trace_page_pool_state_release(pool, page, release_cnt); + + /* In shutdown phase, last page will free pool instance */ + if (flags & PP_FLAG_SHUTDOWN) + page_pool_free_attempt(pool, hold_cnt, release_cnt);I'm curious why you decided to keep the hold_cnt read separate from the call to free attempt? Not a huge deal, and I'm fine with keeping it this way, just curious if you have any functional reason that I missed, or if you just prefer this style? :)
You seem to have missed my explanation in V2 reply: https://lore.kernel.org/all/f671f5da-d9bc-a559-2120-10c3491e6f6d@xxxxxxxxxx/
} EXPORT_SYMBOL(page_pool_release_page);@@ -535,7 +552,7 @@ static bool page_pool_recycle_in_ring(struct page_pool *pool, struct page *page)static bool page_pool_recycle_in_cache(struct page *page, struct page_pool *pool) { - if (unlikely(pool->alloc.count == PP_ALLOC_CACHE_SIZE)) { + if (pool->alloc.count == PP_ALLOC_CACHE_SIZE) { recycle_stat_inc(pool, cache_full); return false; } @@ -546,6 +563,8 @@ static bool page_pool_recycle_in_cache(struct page *page, return true; }+static void page_pool_empty_ring(struct page_pool *pool);+ /* If the page refcnt == 1, this will try to recycle the page. * if PP_FLAG_DMA_SYNC_DEV is set, we'll try to sync the DMA area for * the configured size min(dma_sync_size, pool->max_len). @@ -572,7 +591,8 @@ __page_pool_put_page(struct page_pool *pool, struct page *page, page_pool_dma_sync_for_device(pool, page, dma_sync_size);- if (allow_direct && in_softirq() &&+ /* During PP shutdown, no direct recycle must occur */ + if (likely(allow_direct && in_softirq()) && page_pool_recycle_in_cache(page, pool)) return NULL;@@ -609,6 +629,8 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page,recycle_stat_inc(pool, ring_full); page_pool_return_page(pool, page); } + if (page && pool->p.flags & PP_FLAG_SHUTDOWN) + page_pool_empty_ring(pool); } EXPORT_SYMBOL(page_pool_put_defragged_page);@@ -646,6 +668,9 @@ void page_pool_put_page_bulk(struct page_pool *pool, void **data,recycle_stat_add(pool, ring, i); page_pool_ring_unlock(pool);+ if (pool->p.flags & PP_FLAG_SHUTDOWN)+ page_pool_empty_ring(pool); + /* Hopefully all pages was return into ptr_ring */ if (likely(i == bulk_len)) return; @@ -737,12 +762,18 @@ struct page *page_pool_alloc_frag(struct page_pool *pool, } EXPORT_SYMBOL(page_pool_alloc_frag);+noinlinestatic void page_pool_empty_ring(struct page_pool *pool) { - struct page *page; + struct page *page, *next; + + next = ptr_ring_consume_bh(&pool->ring);/* Empty recycle ring */- while ((page = ptr_ring_consume_bh(&pool->ring))) { + while (next) { + page = next; + next = ptr_ring_consume_bh(&pool->ring); + /* Verify the refcnt invariant of cached pages */ if (!(page_ref_count(page) == 1)) pr_crit("%s() page_pool refcnt %d violation\n", @@ -796,39 +827,36 @@ static void page_pool_scrub(struct page_pool *pool) page_pool_empty_ring(pool); }-static int page_pool_release(struct page_pool *pool)+/* Avoid inlining code to avoid speculative fetching cacheline */ +noinline +static u32 pp_read_hold_cnt(struct page_pool *pool) +{ + return READ_ONCE(pool->pages_state_hold_cnt); +} + +noinline +static int page_pool_free_attempt(struct page_pool *pool, + u32 hold_cnt, u32 release_cnt) { int inflight;- page_pool_scrub(pool);- inflight = page_pool_inflight(pool); + inflight = __page_pool_inflight(pool, hold_cnt, release_cnt); if (!inflight) page_pool_free(pool);return inflight;}-static void page_pool_release_retry(struct work_struct *wq)+static int page_pool_release(struct page_pool *pool) { - struct delayed_work *dwq = to_delayed_work(wq); - struct page_pool *pool = container_of(dwq, typeof(*pool), release_dw); int inflight;- inflight = page_pool_release(pool);+ page_pool_scrub(pool); + inflight = page_pool_inflight(pool); if (!inflight) - return; - - /* Periodic warning */ - if (time_after_eq(jiffies, pool->defer_warn)) { - int sec = (s32)((u32)jiffies - (u32)pool->defer_start) / HZ; - - pr_warn("%s() stalled pool shutdown %d inflight %d sec\n", - __func__, inflight, sec); - pool->defer_warn = jiffies + DEFER_WARN_INTERVAL; - } + page_pool_free(pool);- /* Still not ready to be disconnected, retry later */- schedule_delayed_work(&pool->release_dw, DEFER_TIME); + return inflight; }void page_pool_use_xdp_mem(struct page_pool *pool, void (*disconnect)(void *),@@ -856,6 +884,10 @@ EXPORT_SYMBOL(page_pool_unlink_napi);void page_pool_destroy(struct page_pool *pool){ + unsigned int flags; + u32 release_cnt; + u32 hold_cnt; + if (!pool) return;@@ -868,11 +900,39 @@ void page_pool_destroy(struct page_pool *pool)if (!page_pool_release(pool)) return;- pool->defer_start = jiffies;- pool->defer_warn = jiffies + DEFER_WARN_INTERVAL; + /* PP have pages inflight, thus cannot immediately release memory. + * Enter into shutdown phase, depending on remaining in-flight PP + * pages to trigger shutdown process (on concurrent CPUs) and last + * page will free pool instance. + * + * There exist two race conditions here, we need to take into + * account in the following code. + * + * 1. Before setting PP_FLAG_SHUTDOWN another CPU released the last + * pages into the ptr_ring. Thus, it missed triggering shutdown + * process, which can then be stalled forever. + * + * 2. After setting PP_FLAG_SHUTDOWN another CPU released the last + * page, which triggered shutdown process and freed pool + * instance. Thus, its not safe to dereference *pool afterwards. + * + * Handling races by holding a fake in-flight count, via + * artificially bumping pages_state_hold_cnt, which assures pool + * isn't freed under us. For race(1) its safe to recheck ptr_ring + * (it will not free pool). Race(2) cannot happen, and we can + * release fake in-flight count as last step. + */ + hold_cnt = READ_ONCE(pool->pages_state_hold_cnt) + 1; + smp_store_release(&pool->pages_state_hold_cnt, hold_cnt); + barrier(); + flags = READ_ONCE(pool->p.flags) | PP_FLAG_SHUTDOWN; + smp_store_release(&pool->p.flags, flags);So in the memory barrier documentation, store_release() is usually paired with read_acquire(), but the code reading the flag uses READ_ONCE(). I'm not sure if those are equivalent? (As in, I am asking more than I'm saying they're not; I find it difficult to keep these things straight...)
I was hoping someone could help us say what barrier to use here? --Jesper
