http1: Optionally honor thread_stats_rate

When a session is continuously used, for example in a multi-tier setup,
it can take a very long time before an HTTP/1 session runs out of tasks
and unwinds back to the point where the worker might update its stats.

This phenomenon is amplified by thread_stats_rate, so it might take even
longer to get statistics published. This is famously observed with load
testing campaigns where a certain set of VSCs may not increase at all
until the very end of a load run.

This change makes it possible for HTTP/1 sessions to at least publish
their statistics into the pool every thread_stats_rate requests.

To avoid a potential performance cost, this behavior (technically
complying with the documentation) is guarded by a feature flag. The
flag is generic so this behavior may be extended to other busy loops,
present (if any) or future.

bin/varnishd/cache/cache_varnishd.h

@@ -478,6 +478,7 @@ void VMOD_Panic(struct vsb *);
 
 /* cache_wrk.c */
 void WRK_Init(void);
+void WRK_AddStat(struct worker *);
 
 /* cache_ws.c */
 void WS_Id(const struct ws *ws, char *id);

bin/varnishd/cache/cache_wrk.c

@@ -169,6 +169,25 @@ pool_addstat(struct VSC_main_wrk *dst, struct VSC_main_wrk *src)
 	memset(src, 0, sizeof *src);
 }
 
+void
+WRK_AddStat(struct worker *wrk)
+{
+	struct pool *pp;
+
+	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
+	pp = wrk->pool;
+	CHECK_OBJ_NOTNULL(pp, POOL_MAGIC);
+	if (++wrk->stats->summs >= cache_param->wthread_stats_rate) {
+		Lck_Lock(&pp->mtx);
+		pool_addstat(pp->a_stat, wrk->stats);
+		Lck_Unlock(&pp->mtx);
+	}
+}
+
+/*--------------------------------------------------------------------
+ * Pool reserve calculation
+ */
+
 static unsigned
 pool_reserve(void)
 {

bin/varnishd/http1/cache_http1_fsm.c

@@ -414,6 +414,8 @@ HTTP1_Session(struct worker *wrk, struct req *req)
 			HTC_RxInit(req->htc, req->ws);
 			if (req->htc->rxbuf_e != req->htc->rxbuf_b)
 				wrk->stats->sess_readahead++;
+			if (FEATURE(FEATURE_BUSY_STATS_RATE))
+				WRK_AddStat(wrk);
 			http1_setstate(sp, H1NEWREQ);
 		} else {
 			WRONG("Wrong H1 session state");

include/tbl/feature_bits.h

@@ -78,6 +78,10 @@ FEATURE_BIT(VALIDATE_HEADERS,		validate_headers,
     "Validate all header set operations to conform to RFC7230."
 )
 
+FEATURE_BIT(BUSY_STATS_RATE,	busy_stats_rate,
+    "Make busy workers comply with thread_stats_rate."
+)
+
 #undef FEATURE_BIT
 
 /*lint -restore */