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Subject[RFC PATCH v3 04/12] lib: vdso: inline do_hres() and do_coarse()
Date
do_hres() is called from several places, so GCC doesn't inline
it at first.

do_hres() takes a struct __kernel_timespec * parameter for
passing the result. In the 32 bits case, this parameter corresponds
to a local var in the caller. In order to provide a pointer
to this structure, the caller has to put it in its stack and
do_hres() has to write the result in the stack. This is suboptimal,
especially on RISC processor like powerpc.

By making GCC inline the function, the struct __kernel_timespec
remains a local var using registers, avoiding the need to write and
read stack.

The improvement is significant on powerpc:
Before:
gettimeofday: vdso: 1379 nsec/call
clock-gettime-realtime-coarse: vdso: 868 nsec/call
clock-gettime-realtime: vdso: 1511 nsec/call
clock-gettime-monotonic-raw: vdso: 1576 nsec/call

After:
gettimeofday: vdso: 1078 nsec/call
clock-gettime-realtime-coarse: vdso: 807 nsec/call
clock-gettime-realtime: vdso: 1256 nsec/call
clock-gettime-monotonic-raw: vdso: 1316 nsec/call

At the same time, change the return type of do_coarse() to int, this
increase readability of the if/elseif/elseif/else section
in __cvdso_clock_gettime_common()

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
lib/vdso/gettimeofday.c | 29 ++++++++++++++++-------------
1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index 42bd8ab955fa..d75e44ba716f 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -38,8 +38,8 @@ u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
}
#endif

-static int do_hres(const struct vdso_data *vd, clockid_t clk,
- struct __kernel_timespec *ts)
+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;
@@ -68,8 +68,8 @@ static int do_hres(const struct vdso_data *vd, clockid_t clk,
return 0;
}

-static void do_coarse(const struct vdso_data *vd, clockid_t clk,
- struct __kernel_timespec *ts)
+static __always_inline int do_coarse(const struct vdso_data *vd, clockid_t clk,
+ struct __kernel_timespec *ts)
{
const struct vdso_timestamp *vdso_ts = &vd->basetime[clk];
u32 seq;
@@ -79,6 +79,8 @@ static void do_coarse(const struct vdso_data *vd, clockid_t clk,
ts->tv_sec = vdso_ts->sec;
ts->tv_nsec = vdso_ts->nsec;
} while (unlikely(vdso_read_retry(vd, seq)));
+
+ return 0;
}

static __maybe_unused int
@@ -96,15 +98,16 @@ __cvdso_clock_gettime_common(clockid_t clock, struct __kernel_timespec *ts)
* clocks are handled in the VDSO directly.
*/
msk = 1U << clock;
- if (likely(msk & VDSO_HRES)) {
- return do_hres(&vd[CS_HRES_COARSE], clock, ts);
- } else if (msk & VDSO_COARSE) {
- do_coarse(&vd[CS_HRES_COARSE], clock, ts);
- return 0;
- } else if (msk & VDSO_RAW) {
- return do_hres(&vd[CS_RAW], clock, ts);
- }
- return -1;
+ if (likely(msk & VDSO_HRES))
+ vd += CS_HRES_COARSE;
+ else if (msk & VDSO_COARSE)
+ return do_coarse(&vd[CS_HRES_COARSE], clock, ts);
+ else if (msk & VDSO_RAW)
+ vd += CS_RAW;
+ else
+ return -1;
+
+ return do_hres(vd, clock, ts);
}

static __maybe_unused int
--
2.13.3
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