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Date:   Sun, 22 Mar 2020 11:18:48 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ian Rogers <irogers@google.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        linux-kernel@vger.kernel.org, Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH v2] perf test x86: address multiplexing in rdpmc test
Message-ID: <20200322101848.GF2452@worktop.programming.kicks-ass.net>
References: <20200321173710.127770-1-irogers@google.com>
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On Sat, Mar 21, 2020 at 10:37:10AM -0700, Ian Rogers wrote:

> +static u64 mmap_read_self(void *addr, u64 *running)
>  {
>  	struct perf_event_mmap_page *pc = addr;
>  	u32 seq, idx, time_mult = 0, time_shift = 0;
> -	u64 count, cyc = 0, time_offset = 0, enabled, running, delta;
> +	u64 count, cyc = 0, time_offset = 0, enabled, delta;
>  
>  	do {
>  		seq = pc->lock;
>  		barrier();
>  
>  		enabled = pc->time_enabled;
> -		running = pc->time_running;
> -
> -		if (enabled != running) {
> +		*running = pc->time_running;
> +
> +		if (*running == 0) {
> +			/*
> +			 * Counter won't have a value as due to multiplexing the
> +			 * event wasn't scheduled.
> +			 */
> +			return 0;
> +		}

I still think adding code for an error case here is a bad idea. And only
passing running as an argument is inconsistent.

Also, then I had a look at what the compiler made of that function and
cried.

Here's something a little better. Much of it copied from linux/math64.h
and asm/div64.h.

---
diff --git a/tools/perf/arch/x86/tests/rdpmc.c b/tools/perf/arch/x86/tests/rdpmc.c
index 1ea916656a2d..386a6dacb21e 100644
--- a/tools/perf/arch/x86/tests/rdpmc.c
+++ b/tools/perf/arch/x86/tests/rdpmc.c
@@ -34,20 +34,98 @@ static u64 rdtsc(void)
 	return low | ((u64)high) << 32;
 }

-static u64 mmap_read_self(void *addr)
+#ifdef __x86_64__
+static inline u64 mul_u64_u64_div64(u64 a, u64 b, u64 c)
+{
+	u64 q;
+
+	asm ("mulq %2; divq %3" : "=a" (q)
+				: "a" (a), "rm" (b), "rm" (c)
+				: "rdx");
+
+	return q;
+}
+#define mul_u64_u64_div64 mul_u64_u64_div64
+#endif
+
+#ifdef __SIZEOF_INT128__
+
+static inline u64 mul_u64_u32_shr(u64 a, u32 b, unsigned int shift)
+{
+	return (u64)(((unsigned __int128)a * b) >> shift);
+}
+
+#ifndef mul_u64_u64_div64
+static inline u64 mul_u64_u64_div64(u64 a, u64 b, u64 c)
+{
+	unsigned __int128 m = a;
+	m *= b;
+	return m / c;
+}
+#endif
+
+#else
+
+#ifdef __i386__
+static inline u64 mul_u32_u32(u32 a, u32 b)
+{
+	u32 high, low;
+
+	asm ("mull %[b]" : "=a" (low), "=d" (high)
+			 : [a] "a" (a), [b] "rm" (b) );
+
+	return low | ((u64)high) << 32;
+}
+#else
+static inline u64 mul_u32_u32(u32 a, u32 b)
+{
+	return (u64)a * b;
+}
+#endif
+
+static inline u64 mul_u64_u32_shr(u64 a, u32 b, unsigned int shift)
+{
+	u32 ah, al;
+	u64 ret;
+
+	al = a;
+	ah = a >> 32;
+
+	ret = mul_u32_u32(al, mul) >> shift;
+	if (ah)
+		ret += mul_u32_u32(ah, mul) << (32 - shift);
+
+	return ret;
+}
+
+#ifndef mul_u64_u64_div64
+static inline u64 mul_u64_u64_div64(u64 a, u64 b, u64 c)
+{
+	u64 quot, rem;
+
+	quot = a / c;
+	rem = a % c;
+
+	return qout * b + (rem * b) / c;
+}
+#endif
+
+#endif
+
+static u64 mmap_read_self(void *addr, u64 *enabled, u64 *running)
 {
 	struct perf_event_mmap_page *pc = addr;
 	u32 seq, idx, time_mult = 0, time_shift = 0;
-	u64 count, cyc = 0, time_offset = 0, enabled, running, delta;
+	u64 count, cyc = 0, time_offset = 0;

 	do {
 		seq = pc->lock;
 		barrier();

-		enabled = pc->time_enabled;
-		running = pc->time_running;
+		*enabled = pc->time_enabled;
+		*running = pc->time_running;

-		if (enabled != running) {
+		if (*enabled != *running) {
 			cyc = rdtsc();
 			time_mult = pc->time_mult;
 			time_shift = pc->time_shift;
@@ -62,21 +140,13 @@ static u64 mmap_read_self(void *addr)
 		barrier();
 	} while (pc->lock != seq);

-	if (enabled != running) {
-		u64 quot, rem;
-
-		quot = (cyc >> time_shift);
-		rem = cyc & (((u64)1 << time_shift) - 1);
-		delta = time_offset + quot * time_mult +
-			((rem * time_mult) >> time_shift);
-
-		enabled += delta;
+	if (*enabled != *running) {
+		u64 delta = time_offset + mul_u64_u32_shr(cyc, time_mult, time_shift);
+		*enabled += delta;
 		if (idx)
-			running += delta;
+			*running += delta;

-		quot = count / running;
-		rem = count % running;
-		count = quot * enabled + (rem * enabled) / running;
+		count = mul_u64_u64_div64(count, *enabled, *running);
 	}

 	return count;
@@ -130,14 +200,18 @@ static int __test__rdpmc(void)
 	}

 	for (n = 0; n < 6; n++) {
-		u64 stamp, now, delta;
+		u64 stamp, now, delta, enabled, running;

-		stamp = mmap_read_self(addr);
+		stamp = mmap_read_self(addr, &enabled, &running);

 		for (i = 0; i < loops; i++)
 			tmp++;

-		now = mmap_read_self(addr);
+		now = mmap_read_self(addr, &enabled, &running);
+
+		if (enabled && !running)
+			goto out_error;
+
 		loops *= 10;

 		delta = now - stamp;
@@ -155,6 +229,11 @@ static int __test__rdpmc(void)
 		return -1;

 	return 0;
+
+out_error:
+	close(fd);
+	pr_err("counter never ran; you loose\n");
+	return -1;
 }

 int test__rdpmc(struct test *test __maybe_unused, int subtest __maybe_unused)