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kernel/arch: enhance the "ready thread" cache

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The way the ready thread cache was implemented caused it to not always
be "hot", i.e. there could be some misses, which happened when the
cached thread was taken out of the ready queue. When that happened, it
was not replaced immediately, since doing so could mean that the
replacement might not run because the flow could be interrupted and
another thread could take its place. This was the more conservative
approach that insured that moving a thread to the cache would never be
wasted.

However, this caused two problems:

1. The cache could not be refilled until another thread context-switched
in, since there was no thread in the cache to compare priorities
against.

2. Interrupt exit code would always have to call into C to find what
thread to run when the current thread was not coop and did not have the
scheduler locked. Furthermore, it was possible for this code path to
encounter a cold cache and then it had to find out what thread to run
the long way.

To fix this, filling the cache is now more aggressive, i.e. the next
thread to put in the cache is found even in the case the current cached
thread is context-switched out. This ensures the interrupt exit code is
much faster on the slow path. In addition, since finding the next thread
to run is now always "get it from the cache", which is a simple fetch
from memory (_kernel.ready_q.cache), there is no need to call the more
complex C code.

On the ARM FRDM K64F board, this improvement is seen:

Before:

1- Measure time to switch from ISR back to interrupted task

   switching time is 215 tcs = 1791 nsec

2- Measure time from ISR to executing a different task (rescheduled)

   switch time is 315 tcs = 2625 nsec

After:

1- Measure time to switch from ISR back to interrupted task

   switching time is 130 tcs = 1083 nsec

2- Measure time from ISR to executing a different task (rescheduled)

   switch time is 225 tcs = 1875 nsec

These are the most dramatic improvements, but most of the numbers
generated by the latency_measure test are improved.

Fixes ZEP-1401.

Change-Id: I2eaac147048b1ec71a93bd0a285e743a39533973
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>
This commit is contained in:
Benjamin Walsh 2016-12-02 10:37:27 -05:00 • committed by Benjamin Walsh
commit 88b3691415
17 changed files with 85 additions and 180 deletions
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12
arch/nios2/core/swap.S
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@ -25,7 +25,6 @@ GTEXT(_thread_entry_wrapper)
/* imports */
GTEXT(_sys_k_event_logger_context_switch)
GTEXT(_get_next_ready_thread)
GTEXT(_k_neg_eagain)
/* unsigned int _Swap(unsigned int key)
@ -78,16 +77,17 @@ SECTION_FUNC(exception.other, _Swap)
ori r10, r10, %lo(_kernel)
#endif /* CONFIG_KERNEL_EVENT_LOGGER_CONTEXT_SWITCH */
/* Assign to _kernel.current the return value of
* _get_next_ready_thread()
*/
call _get_next_ready_thread
movhi r10, %hi(_kernel)
ori r10, r10, %lo(_kernel)
stw r2, _kernel_offset_to_current(r10)
/* get cached thread to run */
ldw r2, _kernel_offset_to_ready_q_cache(r10)
/* At this point r2 points to the next thread to be swapped in */
/* the thread to be swapped in is now the current thread */
stw r2, _kernel_offset_to_current(r10)
/* Restore callee-saved registers and switch to the incoming
* thread's stack
*/