Performance: split check deliver fast/slow paths
[lttng-ust.git] / libringbuffer / ring_buffer_frontend.c
index 908539a8663973fde0a8e51384907b4690b0e30d..bda0e1f497fecc786883726b6329ba51498240c9 100644 (file)
@@ -421,6 +421,113 @@ void lib_ring_buffer_channel_switch_timer(int sig, siginfo_t *si, void *uc)
        return;
 }
 
+static
+int lib_ring_buffer_poll_deliver(const struct lttng_ust_lib_ring_buffer_config *config,
+                                struct lttng_ust_lib_ring_buffer *buf,
+                                struct channel *chan,
+                                struct lttng_ust_shm_handle *handle)
+{
+       unsigned long consumed_old, consumed_idx, commit_count, write_offset;
+
+       consumed_old = uatomic_read(&buf->consumed);
+       consumed_idx = subbuf_index(consumed_old, chan);
+       commit_count = v_read(config, &shmp_index(handle, buf->commit_cold, consumed_idx)->cc_sb);
+       /*
+        * No memory barrier here, since we are only interested
+        * in a statistically correct polling result. The next poll will
+        * get the data is we are racing. The mb() that ensures correct
+        * memory order is in get_subbuf.
+        */
+       write_offset = v_read(config, &buf->offset);
+
+       /*
+        * Check that the subbuffer we are trying to consume has been
+        * already fully committed.
+        */
+
+       if (((commit_count - chan->backend.subbuf_size)
+            & chan->commit_count_mask)
+           - (buf_trunc(consumed_old, chan)
+              >> chan->backend.num_subbuf_order)
+           != 0)
+               return 0;
+
+       /*
+        * Check that we are not about to read the same subbuffer in
+        * which the writer head is.
+        */
+       if (subbuf_trunc(write_offset, chan) - subbuf_trunc(consumed_old, chan)
+           == 0)
+               return 0;
+
+       return 1;
+}
+
+static
+void lib_ring_buffer_wakeup(struct lttng_ust_lib_ring_buffer *buf,
+               struct lttng_ust_shm_handle *handle)
+{
+       int wakeup_fd = shm_get_wakeup_fd(handle, &buf->self._ref);
+       sigset_t sigpipe_set, pending_set, old_set;
+       int ret, sigpipe_was_pending = 0;
+
+       if (wakeup_fd < 0)
+               return;
+
+       /*
+        * Wake-up the other end by writing a null byte in the pipe
+        * (non-blocking).  Important note: Because writing into the
+        * pipe is non-blocking (and therefore we allow dropping wakeup
+        * data, as long as there is wakeup data present in the pipe
+        * buffer to wake up the consumer), the consumer should perform
+        * the following sequence for waiting:
+        * 1) empty the pipe (reads).
+        * 2) check if there is data in the buffer.
+        * 3) wait on the pipe (poll).
+        *
+        * Discard the SIGPIPE from write(), not disturbing any SIGPIPE
+        * that might be already pending. If a bogus SIGPIPE is sent to
+        * the entire process concurrently by a malicious user, it may
+        * be simply discarded.
+        */
+       ret = sigemptyset(&pending_set);
+       assert(!ret);
+       /*
+        * sigpending returns the mask of signals that are _both_
+        * blocked for the thread _and_ pending for either the thread or
+        * the entire process.
+        */
+       ret = sigpending(&pending_set);
+       assert(!ret);
+       sigpipe_was_pending = sigismember(&pending_set, SIGPIPE);
+       /*
+        * If sigpipe was pending, it means it was already blocked, so
+        * no need to block it.
+        */
+       if (!sigpipe_was_pending) {
+               ret = sigemptyset(&sigpipe_set);
+               assert(!ret);
+               ret = sigaddset(&sigpipe_set, SIGPIPE);
+               assert(!ret);
+               ret = pthread_sigmask(SIG_BLOCK, &sigpipe_set, &old_set);
+               assert(!ret);
+       }
+       do {
+               ret = write(wakeup_fd, "", 1);
+       } while (ret == -1L && errno == EINTR);
+       if (ret == -1L && errno == EPIPE && !sigpipe_was_pending) {
+               struct timespec timeout = { 0, 0 };
+               do {
+                       ret = sigtimedwait(&sigpipe_set, NULL,
+                               &timeout);
+               } while (ret == -1L && errno == EINTR);
+       }
+       if (!sigpipe_was_pending) {
+               ret = pthread_sigmask(SIG_SETMASK, &old_set, NULL);
+               assert(!ret);
+       }
+}
+
 static
 void lib_ring_buffer_channel_do_read(struct channel *chan)
 {
@@ -2049,6 +2156,130 @@ int lib_ring_buffer_reserve_slow(struct lttng_ust_lib_ring_buffer_ctx *ctx)
        return 0;
 }
 
+static
+void lib_ring_buffer_vmcore_check_deliver(const struct lttng_ust_lib_ring_buffer_config *config,
+                                         struct lttng_ust_lib_ring_buffer *buf,
+                                         unsigned long commit_count,
+                                         unsigned long idx,
+                                         struct lttng_ust_shm_handle *handle)
+{
+       if (config->oops == RING_BUFFER_OOPS_CONSISTENCY)
+               v_set(config, &shmp_index(handle, buf->commit_hot, idx)->seq, commit_count);
+}
+
+void lib_ring_buffer_check_deliver_slow(const struct lttng_ust_lib_ring_buffer_config *config,
+                                  struct lttng_ust_lib_ring_buffer *buf,
+                                  struct channel *chan,
+                                  unsigned long offset,
+                                  unsigned long commit_count,
+                                  unsigned long idx,
+                                  struct lttng_ust_shm_handle *handle,
+                                  uint64_t tsc)
+{
+       unsigned long old_commit_count = commit_count
+                                        - chan->backend.subbuf_size;
+
+       /*
+        * If we succeeded at updating cc_sb below, we are the subbuffer
+        * writer delivering the subbuffer. Deals with concurrent
+        * updates of the "cc" value without adding a add_return atomic
+        * operation to the fast path.
+        *
+        * We are doing the delivery in two steps:
+        * - First, we cmpxchg() cc_sb to the new value
+        *   old_commit_count + 1. This ensures that we are the only
+        *   subbuffer user successfully filling the subbuffer, but we
+        *   do _not_ set the cc_sb value to "commit_count" yet.
+        *   Therefore, other writers that would wrap around the ring
+        *   buffer and try to start writing to our subbuffer would
+        *   have to drop records, because it would appear as
+        *   non-filled.
+        *   We therefore have exclusive access to the subbuffer control
+        *   structures.  This mutual exclusion with other writers is
+        *   crucially important to perform record overruns count in
+        *   flight recorder mode locklessly.
+        * - When we are ready to release the subbuffer (either for
+        *   reading or for overrun by other writers), we simply set the
+        *   cc_sb value to "commit_count" and perform delivery.
+        *
+        * The subbuffer size is least 2 bytes (minimum size: 1 page).
+        * This guarantees that old_commit_count + 1 != commit_count.
+        */
+
+       /*
+        * Order prior updates to reserve count prior to the
+        * commit_cold cc_sb update.
+        */
+       cmm_smp_wmb();
+       if (caa_likely(v_cmpxchg(config, &shmp_index(handle, buf->commit_cold, idx)->cc_sb,
+                                old_commit_count, old_commit_count + 1)
+                  == old_commit_count)) {
+               /*
+                * Start of exclusive subbuffer access. We are
+                * guaranteed to be the last writer in this subbuffer
+                * and any other writer trying to access this subbuffer
+                * in this state is required to drop records.
+                */
+               v_add(config,
+                     subbuffer_get_records_count(config,
+                                                 &buf->backend,
+                                                 idx, handle),
+                     &buf->records_count);
+               v_add(config,
+                     subbuffer_count_records_overrun(config,
+                                                     &buf->backend,
+                                                     idx, handle),
+                     &buf->records_overrun);
+               config->cb.buffer_end(buf, tsc, idx,
+                                     lib_ring_buffer_get_data_size(config,
+                                                               buf,
+                                                               idx,
+                                                               handle),
+                                     handle);
+
+               /*
+                * Increment the packet counter while we have exclusive
+                * access.
+                */
+               subbuffer_inc_packet_count(config, &buf->backend, idx, handle);
+
+               /*
+                * Set noref flag and offset for this subbuffer id.
+                * Contains a memory barrier that ensures counter stores
+                * are ordered before set noref and offset.
+                */
+               lib_ring_buffer_set_noref_offset(config, &buf->backend, idx,
+                                                buf_trunc_val(offset, chan), handle);
+
+               /*
+                * Order set_noref and record counter updates before the
+                * end of subbuffer exclusive access. Orders with
+                * respect to writers coming into the subbuffer after
+                * wrap around, and also order wrt concurrent readers.
+                */
+               cmm_smp_mb();
+               /* End of exclusive subbuffer access */
+               v_set(config, &shmp_index(handle, buf->commit_cold, idx)->cc_sb,
+                     commit_count);
+               /*
+                * Order later updates to reserve count after
+                * the commit cold cc_sb update.
+                */
+               cmm_smp_wmb();
+               lib_ring_buffer_vmcore_check_deliver(config, buf,
+                                        commit_count, idx, handle);
+
+               /*
+                * RING_BUFFER_WAKEUP_BY_WRITER wakeup is not lock-free.
+                */
+               if (config->wakeup == RING_BUFFER_WAKEUP_BY_WRITER
+                   && uatomic_read(&buf->active_readers)
+                   && lib_ring_buffer_poll_deliver(config, buf, chan, handle)) {
+                       lib_ring_buffer_wakeup(buf, handle);
+               }
+       }
+}
+
 /*
  * Force a read (imply TLS fixup for dlopen) of TLS variables.
  */
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