Skip checksum benchmarks on systems with slow cpu

The checksum benchmarking on module load may take a really long time
on embedded systems with a slow cpu. Avoid all benchmarks >= 1MiB on
systems, where EdonR is slower then 300 MiB/s.

This limit is currently hardcoded, but could also be a tuneable.
When we switch to a tuneable, what name should I use then?

This is the new benchmark output of a slow Intel Atom:

```
 implementation    1k    4k   16k   64k  256k
 edonr-generic    209   257   268   259   262
 skein-generic    129   150   151   150   150
 sha256-generic    50    55    56    56    56
 sha512-generic    76    86    88    89    88
 blake3-generic    63    62    62    62    61
 blake3-sse2      114   292   301   307   309
```

Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>

module/zfs/zfs_chksum.c

@@ -31,7 +31,8 @@
 
 #include <sys/blake3.h>
 
-static kstat_t *chksum_kstat = NULL;
+/* limit benchmarking to max 256KiB, when EdonR is slower then this: */
+#define	LIMIT_PERF_MBS	300
 
 typedef struct {
 	const char *name;
@@ -50,8 +51,9 @@ typedef struct {
 	zio_checksum_tmpl_free_t *(free);
 } chksum_stat_t;
 
-static int chksum_stat_cnt = 0;
 static chksum_stat_t *chksum_stat_data = 0;
+static int chksum_stat_cnt = 0;
+static kstat_t *chksum_kstat = NULL;
 
 /*
  * i3-1005G1 test output:
@@ -75,7 +77,7 @@ static chksum_stat_t *chksum_stat_data = 0;
  * blake3-avx512     473    2687    4905    5836    5844    5643    5374
  */
 static int
-chksum_stat_kstat_headers(char *buf, size_t size)
+chksum_kstat_headers(char *buf, size_t size)
 {
 	ssize_t off = 0;
 
@@ -93,7 +95,7 @@ chksum_stat_kstat_headers(char *buf, size_t size)
 }
 
 static int
-chksum_stat_kstat_data(char *buf, size_t size, void *data)
+chksum_kstat_data(char *buf, size_t size, void *data)
 {
 	chksum_stat_t *cs;
 	ssize_t off = 0;
@@ -123,7 +125,7 @@ chksum_stat_kstat_data(char *buf, size_t size, void *data)
 }
 
 static void *
-chksum_stat_kstat_addr(kstat_t *ksp, loff_t n)
+chksum_kstat_addr(kstat_t *ksp, loff_t n)
 {
 	if (n < chksum_stat_cnt)
 		ksp->ks_private = (void *)(chksum_stat_data + n);
@@ -176,17 +178,21 @@ chksum_run(chksum_stat_t *cs, abd_t *abd, void *ctx, int round,
 	*result = run_bw/1024/1024; /* MiB/s */
 }
 
+#define	LIMIT_INIT	0
+#define	LIMIT_NEEDED	1
+#define	LIMIT_NOLIMIT	2
+
 static void
 chksum_benchit(chksum_stat_t *cs)
 {
 	abd_t *abd;
 	void *ctx = 0;
 	void *salt = &cs->salt.zcs_bytes;
+	static int chksum_stat_limit = LIMIT_INIT;
 
 	memset(salt, 0, sizeof (cs->salt.zcs_bytes));
-	if (cs->init) {
+	if (cs->init)
 		ctx = cs->init(&cs->salt);
-	}
 
 	/* allocate test memory via abd linear interface */
 	abd = abd_alloc_linear(1<<20, B_FALSE);
@@ -195,19 +201,34 @@ chksum_benchit(chksum_stat_t *cs)
 	chksum_run(cs, abd, ctx, 3, &cs->bs16k);
 	chksum_run(cs, abd, ctx, 4, &cs->bs64k);
 	chksum_run(cs, abd, ctx, 5, &cs->bs256k);
+
+	/* check if we ran on a slow cpu */
+	if (chksum_stat_limit == LIMIT_INIT) {
+		if (cs->bs1k < 300) {
+			chksum_stat_limit = LIMIT_NEEDED;
+		} else {
+			chksum_stat_limit = LIMIT_NOLIMIT;
+		}
+	}
+
+	/* skip benchmarks >= 1MiB when the CPU is to slow */
+	if (chksum_stat_limit == LIMIT_NEEDED)
+		goto abort;
+
 	chksum_run(cs, abd, ctx, 6, &cs->bs1m);
 	abd_free(abd);
 
 	/* allocate test memory via abd non linear interface */
 	abd = abd_alloc(1<<24, B_FALSE);
 	chksum_run(cs, abd, ctx, 7, &cs->bs4m);
 	chksum_run(cs, abd, ctx, 8, &cs->bs16m);
+
+abort:
 	abd_free(abd);
 
 	/* free up temp memory */
-	if (cs->free) {
+	if (cs->free)
 		cs->free(ctx);
-	}
 }
 
 /*
@@ -232,7 +253,7 @@ chksum_benchmark(void)
 	chksum_stat_data = (chksum_stat_t *)kmem_zalloc(
 	    sizeof (chksum_stat_t) * chksum_stat_cnt, KM_SLEEP);
 
-	/* edonr */
+	/* edonr - needs to be the first one here (slow CPU check) */
 	cs = &chksum_stat_data[cbid++];
 	cs->init = abd_checksum_edonr_tmpl_init;
 	cs->func = abd_checksum_edonr_native;
@@ -303,9 +324,9 @@ chksum_init(void)
 		chksum_kstat->ks_data = NULL;
 		chksum_kstat->ks_ndata = UINT32_MAX;
 		kstat_set_raw_ops(chksum_kstat,
-		    chksum_stat_kstat_headers,
-		    chksum_stat_kstat_data,
-		    chksum_stat_kstat_addr);
+		    chksum_kstat_headers,
+		    chksum_kstat_data,
+		    chksum_kstat_addr);
 		kstat_install(chksum_kstat);
 	}