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SubjectRe: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY
On 2008-11-28 14:10, Nick Piggin wrote:
> This is what I have.
>
> It does two things. Firstly, it switches x86-64 over to use the xadd
> algorithm rather than the spinlock algorithm. This is actually significant
> in high contention situations, because the spinlock algorithm doesn't allow
> concurrent operations on the lock while the queue of waiters is being
> manipulated.
>
> Secondly, it moves wakeups out from underneath the waiter queue lock. This
> is more significant on bigger machines where wakeup latency is worse and/or
> runqueue locks are very heavily contended.
>
> Now both these changes are going to help *mainly* for the case when there are
> a significant number of readers and writers, I think. So your write-heavy
> workload may not win anything. I noticed some speedup a long time ago on
> some weird java (volanomark) workload.

Hi,

I just tested your patch on top of tip/master, and my testprogram has
segfaulted :(
It is either something wrong in tip/master or the patch, or my program.
This is the first time this testprogram segfaults, and it doesn't have a
reason to segfault there.


[ 140.624155] scalability[4995]: segfault at 7f9ce137f000 ip
0000000000401a62 sp 00000000454950a0 error 4 in scalability[400000+3000]
[ 401.640738] scalability[5398]: segfault at 7fdbffba3000 ip
0000000000401a62 sp 00000000423d70a0 error 4 in scalability[400000+3000]

Here is the relevant portion, at 401a62 I read from the mapping:

static void mmap_worker_fn(int fd, off_t len)
{
char *data = mmap(NULL, len, PROT_READ, MAP_PRIVATE, fd, 0);
401a4f: 48 89 c7 mov %rax,%rdi
if(data == MAP_FAILED) {
401a52: 74 36 je 401a8a <mmap_worker_fn+0x5a>
perror("mmap");
abort();
401a54: 31 d2 xor %edx,%edx
401a56: 31 c9 xor %ecx,%ecx
static pthread_mutex_t thrtime_mtx = PTHREAD_MUTEX_INITIALIZER;

static size_t execute(const char *data, size_t len)
{
size_t sum = 0, i;
for(i=0;i<len;++i)
401a58: 48 85 db test %rbx,%rbx
401a5b: 74 28 je 401a85 <mmap_worker_fn+0x55>
401a5d: 0f 1f 00 nopl (%rax)
if(data[i] == 'd')
++sum;
401a60: 31 c0 xor %eax,%eax
401a62: 80 3c 17 64 cmpb $0x64,(%rdi,%rdx,1)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This simply reads from the mapping

401a66: 0f 94 c0 sete %al
static pthread_mutex_t thrtime_mtx = PTHREAD_MUTEX_INITIALIZER;

Steps to reproduce:
# sync; echo 3 >/proc/sys/vm/drop_caches; sync
# echo 0 >/proc/lock_stat
$ sudo ./scalability 16 /usr/bin/
... prints out results for read, and while running mmap_worker ...
... a message about segmentation fault ....

The testprogram is available here:
http://edwintorok.googlepages.com/tst.tar.gz

My .config:
http://edwintorok.googlepages.com/config

Can you reproduce the crash on your box?
Can I help debugging the problem?

Best regards,
--Edwin


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