From: Nicolai Stange <nicstange@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
x86@kernel.org, John Stultz <john.stultz@linaro.org>,
Borislav Petkov <bp@suse.de>, Paolo Bonzini <pbonzini@redhat.com>,
Viresh Kumar <viresh.kumar@linaro.org>,
Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
"Christopher S. Hall" <christopher.s.hall@intel.com>,
Adrian Hunter <adrian.hunter@intel.com>,
Suresh Siddha <suresh.b.siddha@intel.com>,
linux-kernel@vger.kernel.org,
Nicolai Stange <nicstange@gmail.com>
Subject: [PATCH 2/4] arch, x86, tsc deadline clockevent dev: reduce TSC_DIVISOR to 2
Date: Sun, 10 Jul 2016 14:23:43 +0200 [thread overview]
Message-ID: <20160710122345.13061-3-nicstange@gmail.com> (raw)
In-Reply-To: <20160710122345.13061-1-nicstange@gmail.com>
In order to avoid overflowing an u32, the TSC deadline clockevent device's
frequency is divided by TSC_DIVISOR at registration.
The TSC_DIVISOR is currently defined as equaling 32 which allows for a
TSC frequency as high as 2^32 / 10^9ns * 32 = 137 GHz.
OTOH, larger values of TSC_DIVISOR introduce bigger roundoff errors into
the device's frequency.
A value of 2 for TSC_DIVISOR allows for a TSC frequency of
2^32 / 10^9ns * 2 = 8.5 GHz which is still way larger than anything to
expect in the next years.
Reduce the TSC deadline clockevent device's TSC_DIVISOR from 32 down to 2.
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
arch/x86/kernel/apic/apic.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index dce654c..1d22c72 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -311,7 +311,7 @@ int lapic_get_maxlvt(void)
/* Clock divisor */
#define APIC_DIVISOR 16
-#define TSC_DIVISOR 32
+#define TSC_DIVISOR 2
/*
* This function sets up the local APIC timer, with a timeout of
--
2.9.0
next prev parent reply other threads:[~2016-07-10 12:25 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-10 12:23 [PATCH 0/4] avoid double timer interrupt with nohz and Intel TSC Nicolai Stange
2016-07-10 12:23 ` [PATCH 1/4] arch, x86, tsc deadline clockevent dev: reduce frequency roundoff error Nicolai Stange
2016-07-10 12:23 ` Nicolai Stange [this message]
2016-07-10 12:23 ` [PATCH 3/4] arch, x86, tsc: inform TSC deadline clockevent device about recalibration Nicolai Stange
2016-07-10 12:46 ` kbuild test robot
2016-07-10 12:23 ` [PATCH 4/4] kernel/time/clockevents: compensate for monotonic clock's dynamic frequency Nicolai Stange
2016-07-10 13:20 ` kbuild test robot
2016-07-10 13:56 ` kbuild test robot
2016-07-10 14:17 ` kbuild test robot
2016-07-10 19:32 ` [PATCH 0/4] avoid double timer interrupt with nohz and Intel TSC Nicolai Stange
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