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From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	arnd@arndb.de, tglx@linutronix.de, vincenzo.frascino@arm.com,
	luto@kernel.org
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
	x86@kernel.org
Subject: [RFC PATCH v3 06/12] lib: vdso: __iter_div_u64_rem() is suboptimal for 32 bit time
Date: Mon, 13 Jan 2020 17:08:44 +0000 (UTC)
Message-ID: <5b38617a2ca4f719760aafbdb6115eaad28c0640.1578934751.git.christophe.leroy@c-s.fr> (raw)
In-Reply-To: <cover.1578934751.git.christophe.leroy@c-s.fr>

Using __iter_div_ulong_rem() is suboptimal on 32 bits.
Nanoseconds are only 32 bits, and VDSO data is updated every 10ms
so nsec will never overflow 32 bits.

Add an equivalent of __iter_div_u64_rem() but based
on unsigned long to better fit with 32 bits arches.

Before:
gettimeofday:    vdso: 1078 nsec/call
clock-gettime-monotonic-raw:    vdso: 1317 nsec/call
clock-gettime-monotonic:    vdso: 1255 nsec/call

After:
gettimeofday:    vdso: 1032 nsec/call
clock-gettime-monotonic-raw:    vdso: 1312 nsec/call
clock-gettime-monotonic:    vdso: 1243 nsec/call
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
 lib/vdso/gettimeofday.c | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index decd3f2b37af..da15a8842825 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -38,12 +38,32 @@ u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
 }
 #endif
 
+static __always_inline u32
+__iter_div_ulong_rem(unsigned long dividend, u32 divisor, unsigned long *remainder)
+{
+	u32 ret = 0;
+
+	while (dividend >= divisor) {
+		/* The following asm() prevents the compiler from
+		   optimising this loop into a modulo operation.  */
+		asm("" : "+rm"(dividend));
+
+		dividend -= divisor;
+		ret++;
+	}
+
+	*remainder = dividend;
+
+	return ret;
+}
+
 static __always_inline int do_hres(const struct vdso_data *vd, clockid_t clk,
 				   struct __kernel_timespec *ts)
 {
 	const struct vdso_timestamp *vdso_ts = &vd->basetime[clk];
 	u64 cycles, last, sec, ns;
 	u32 seq;
+	unsigned long nsec;
 
 	do {
 		seq = vdso_read_begin(vd);
@@ -54,7 +74,7 @@ static __always_inline int do_hres(const struct vdso_data *vd, clockid_t clk,
 			return -1;
 
 		ns += vdso_calc_delta(cycles, last, vd->mask, vd->mult);
-		ns >>= vd->shift;
+		nsec = ns >> vd->shift;
 		sec = vdso_ts->sec;
 	} while (unlikely(vdso_read_retry(vd, seq)));
 
@@ -62,8 +82,8 @@ static __always_inline int do_hres(const struct vdso_data *vd, clockid_t clk,
 	 * Do this outside the loop: a race inside the loop could result
 	 * in __iter_div_u64_rem() being extremely slow.
 	 */
-	ts->tv_sec = sec + __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
-	ts->tv_nsec = ns;
+	ts->tv_sec = sec + __iter_div_ulong_rem(nsec, NSEC_PER_SEC, &nsec);
+	ts->tv_nsec = nsec;
 
 	return 0;
 }
-- 
2.13.3


  parent reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-13 17:08 [RFC PATCH v3 00/12] powerpc: switch VDSO to C implementation Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 01/12] powerpc/64: Don't provide time functions in compat VDSO32 Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 02/12] powerpc/vdso: Switch VDSO to generic C implementation Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 03/12] lib: vdso: mark __cvdso_clock_getres() as static Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 04/12] lib: vdso: inline do_hres() and do_coarse() Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 05/12] lib: vdso: Avoid duplication in __cvdso_clock_getres() Christophe Leroy
2020-01-13 17:08 ` Christophe Leroy [this message]
2020-01-14 11:31   ` [RFC PATCH v3 06/12] lib: vdso: __iter_div_u64_rem() is suboptimal for 32 bit time Thomas Gleixner
2020-01-13 17:08 ` [RFC PATCH v3 07/12] powerpc/vdso: simplify __get_datapage() Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 08/12] lib: vdso: allow arches to provide vdso data pointer Christophe Leroy
2020-01-14 23:06   ` Thomas Gleixner
2020-01-15  6:15     ` Christophe Leroy
2020-01-16  9:16       ` Christophe Leroy
2020-01-16 10:35         ` Thomas Gleixner
2020-01-16 20:22           ` Andy Lutomirski
2020-01-13 17:08 ` [RFC PATCH v3 09/12] powerpc/vdso: provide inline alternative to __get_datapage() Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 10/12] powerpc/vdso: provide vdso data pointer from the ASM caller Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 11/12] lib: vdso: split clock verification out of __arch_get_hw_counter() Christophe Leroy
2020-01-13 17:08 ` [RFC PATCH v3 12/12] powerpc/vdso: provide __arch_is_hw_counter_valid() Christophe Leroy

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