This is a note to let you know that I've just added the patch titled
parisc: Reduce irq overhead when run in qemu
to the 4.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: parisc-reduce-irq-overhead-when-run-in-qemu.patch and it can be found in the queue-4.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From 636a415bcc7f4fd020ece8fd5fc648c4cef19c34 Mon Sep 17 00:00:00 2001
From: Helge Deller deller@gmx.de Date: Mon, 12 Feb 2018 21:43:55 +0100 Subject: parisc: Reduce irq overhead when run in qemu
From: Helge Deller deller@gmx.de
commit 636a415bcc7f4fd020ece8fd5fc648c4cef19c34 upstream.
When run under QEMU, calling mfctl(16) creates some overhead because the qemu timer has to be scaled and moved into the register. This patch reduces the number of calls to mfctl(16) by moving the calls out of the loops.
Additionally, increase the minimal time interval to 8000 cycles instead of 500 to compensate possible QEMU delays when delivering interrupts.
Signed-off-by: Helge Deller deller@gmx.de Cc: stable@vger.kernel.org # 4.14+ Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/parisc/kernel/time.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-)
--- a/arch/parisc/kernel/time.c +++ b/arch/parisc/kernel/time.c @@ -76,10 +76,10 @@ irqreturn_t __irq_entry timer_interrupt( next_tick = cpuinfo->it_value;
/* Calculate how many ticks have elapsed. */ + now = mfctl(16); do { ++ticks_elapsed; next_tick += cpt; - now = mfctl(16); } while (next_tick - now > cpt);
/* Store (in CR16 cycles) up to when we are accounting right now. */ @@ -103,16 +103,17 @@ irqreturn_t __irq_entry timer_interrupt( * if one or the other wrapped. If "now" is "bigger" we'll end up * with a very large unsigned number. */ - while (next_tick - mfctl(16) > cpt) + now = mfctl(16); + while (next_tick - now > cpt) next_tick += cpt;
/* Program the IT when to deliver the next interrupt. * Only bottom 32-bits of next_tick are writable in CR16! * Timer interrupt will be delivered at least a few hundred cycles - * after the IT fires, so if we are too close (<= 500 cycles) to the + * after the IT fires, so if we are too close (<= 8000 cycles) to the * next cycle, simply skip it. */ - if (next_tick - mfctl(16) <= 500) + if (next_tick - now <= 8000) next_tick += cpt; mtctl(next_tick, 16);
Patches currently in stable-queue which might be from deller@gmx.de are
queue-4.15/parisc-hide-virtual-kernel-memory-layout.patch queue-4.15/parisc-use-cr16-interval-timers-unconditionally-on-qemu.patch queue-4.15/parisc-fix-ordering-of-cache-and-tlb-flushes.patch queue-4.15/parisc-reduce-irq-overhead-when-run-in-qemu.patch