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: x86@kernel.org, linuxppc-dev@lists.ozlabs.org,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, linux-mips@vger.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) [thread overview]
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
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next prev parent reply other threads:[~2020-01-13 17:10 UTC|newest]
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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