From: Nathan Lynch <nath...@linux.ibm.com>
The kernel can handle retrying RTAS function calls in response to
-2/990x in the sys_rtas() handler instead of relaying the intermediate
status to user space.
Justifications:
* Currently it's nondeterministic and quite variable in practice
whether a retry status is returned for any given invocation of
sys_rtas(). Therefore user space code cannot be expecting a retry
result without already being broken.
* This tends to significantly reduce the total number of system calls
issued by programs such as drmgr which make use of sys_rtas(),
improving the experience of tracing and debugging such
programs. This is the main motivation for me: I think this change
will make it easier for us to characterize current sys_rtas() use
cases as we move them to other interfaces over time.
* It reduces the number of opportunities for user space to leave
complex operations, such as those associated with DLPAR, incomplete
and diffcult to recover.
* We can expect performance improvements for existing sys_rtas()
users, not only because of overall reduction in the number of system
calls issued, but also due to the better handling of -2/990x in the
kernel. For example, librtas still sleeps for 1ms on -2, which is
completely unnecessary.
Performance differences for PHB add and remove on a small P10 PowerVM
partition are included below. For add, elapsed time is slightly
reduced. For remove, there are more significant improvements: the
number of context switches is reduced by an order of magnitude, and
elapsed time is reduced by over half.
(- before, + after):
Performance counter stats for 'drmgr -c phb -a -s PHB 23' (5 runs):
- 1,847.58 msec task-clock # 0.135 CPUs
utilized ( +- 14.15% )
- 10,867 cs # 9.800 K/sec
( +- 14.14% )
+ 1,901.15 msec task-clock # 0.148 CPUs
utilized ( +- 14.13% )
+ 10,451 cs # 9.158 K/sec
( +- 14.14% )
- 13.656557 +- 0.000124 seconds time elapsed ( +- 0.00% )
+ 12.88080 +- 0.00404 seconds time elapsed ( +- 0.03% )
Performance counter stats for 'drmgr -c phb -r -s PHB 23' (5 runs):
- 1,473.75 msec task-clock # 0.092 CPUs
utilized ( +- 14.15% )
- 2,652 cs # 3.000 K/sec
( +- 14.16% )
+ 1,444.55 msec task-clock # 0.221 CPUs
utilized ( +- 14.14% )
+ 104 cs # 119.957 /sec
( +- 14.63% )
- 15.99718 +- 0.00801 seconds time elapsed ( +- 0.05% )
+ 6.54256 +- 0.00830 seconds time elapsed ( +- 0.13% )
Move the existing rtas_lock-guarded critical section in sys_rtas()
into a conventional rtas_busy_delay()-based loop, returning to user
space only when a final success or failure result is available.
Signed-off-by: Nathan Lynch <nath...@linux.ibm.com>
---
arch/powerpc/kernel/rtas.c | 28 ++++++++++++++++------------
1 file changed, 16 insertions(+), 12 deletions(-)
diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index 47a2aa43d7d4..c330a22ccc70 100644
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -1798,7 +1798,6 @@ static bool block_rtas_call(int token, int nargs,
/* We assume to be passed big endian arguments */
SYSCALL_DEFINE1(rtas, struct rtas_args __user *, uargs)
{
- struct pin_cookie cookie;
struct rtas_args args;
unsigned long flags;
char *buff_copy, *errbuf = NULL;
@@ -1866,20 +1865,25 @@ SYSCALL_DEFINE1(rtas, struct rtas_args __user *, uargs)
buff_copy = get_errorlog_buffer();
- raw_spin_lock_irqsave(&rtas_lock, flags);
- cookie = lockdep_pin_lock(&rtas_lock);
+ do {
+ struct pin_cookie cookie;
- rtas_args = args;
- do_enter_rtas(&rtas_args);
- args = rtas_args;
+ raw_spin_lock_irqsave(&rtas_lock, flags);
+ cookie = lockdep_pin_lock(&rtas_lock);
- /* A -1 return code indicates that the last command couldn't
- be completed due to a hardware error. */
- if (be32_to_cpu(args.rets[0]) == -1)
- errbuf = __fetch_rtas_last_error(buff_copy);
+ rtas_args = args;
+ do_enter_rtas(&rtas_args);
+ args = rtas_args;
- lockdep_unpin_lock(&rtas_lock, cookie);
- raw_spin_unlock_irqrestore(&rtas_lock, flags);
+ /*
+ * Handle error record retrieval before releasing the lock.
+ */
+ if (be32_to_cpu(args.rets[0]) == -1)
+ errbuf = __fetch_rtas_last_error(buff_copy);
+
+ lockdep_unpin_lock(&rtas_lock, cookie);
+ raw_spin_unlock_irqrestore(&rtas_lock, flags);
+ } while (rtas_busy_delay(be32_to_cpu(args.rets[0])));
if (buff_copy) {
if (errbuf)
--
2.39.1
