rngd: prioritize faster sources of entropy

We have two broad categories of entropy sources in rngd.  Fast sources
and slow sources.  Slow sources (like jitterentropy) provided entropy to
systems that have no other entropy source, but can actually hinder
performance when large amounts of entropy are requested (consider a case
where the entropy pool is empty, and the rdrand source can provide
twice the entropy in half the time that jitterentropy can).  Its still
valuable to have jitterentropy enabled, but we shouldn't be extracting
entropy from it, if a faster source can provide it.  So lets prioritize
our fast sources over the slow ones.  in the do_loop, lets by default
not collect entropy from sources marked as slow.  If we pass
through an interation of the do_loop with no entropy collected, then
lets include the slow sources on the next pass to give us a better
chance at collection.  Then, when entropy is gathered, we can go back to
only using the fast sources.

Signed-off-by: Neil Horman <[email protected]>

rngd.c

@@ -577,6 +577,7 @@ static void do_loop(int random_step)
 	int i;
 	int retval;
 	struct rng *iter;
+	bool try_slow_sources = false;
 
 	int (*random_add_fn)(struct rng *rng, int random_step,
 		unsigned char *buf, fips_ctx_t *fipsctx_in);
@@ -587,11 +588,29 @@ static void do_loop(int random_step)
 	for (no_work = 0; no_work < 100; no_work = (work_done ? 0 : no_work+1)) {
 
 		work_done = false;
+
+		/*
+		 * Exclude slow sources when faster sources are working well
+		 * sources like jitterentropy can provide some entropy when needed
+		 * but can actually hinder performance when large amounts of entropy are needed
+		 * owing to the fact that they may block while generating said entropy
+		 * So, lets prioritize the faster sources.  Start by only trying to collect
+		 * entropy from the fast sources, then iff that fails, start including the slower
+		 * sources as well.  Once we get some entropy, return to only using fast sources
+		 */
+		if (no_work)
+			try_slow_sources = true;
+		else
+			try_slow_sources = false;
+
 		for (i = 0; i < ENT_MAX; ++i)
 		{
 			int rc;
 			/*message(LOG_CONS|LOG_INFO, "I is %d\n", i);*/
 			iter = &entropy_sources[i];
+			if (!try_slow_sources && iter->flags.slow_source)
+				continue;
+
 		retry_same:
 			if (!server_running)
 				return;