* Functions that require interrupts be enabled
@ 2007-05-15 22:33 Mike Mason
2007-05-16 0:49 ` Frank Ch. Eigler
0 siblings, 1 reply; 4+ messages in thread
From: Mike Mason @ 2007-05-15 22:33 UTC (permalink / raw)
To: SystemTAP
I want to add a function to the task.stp tapset file that grabs a process' arguments from its user address space. We couldn't do this before because all probes ran with interrupts disabled and couldn't sleep. Now that begin/end probes no longer require that interrupts be disabled, this function can be used in begin/end probes at least.
How do I prevent the function from being used in other probes? Is there a way to detect if interrupts are disabled or detect that the function was called from a begin/end probe? Do we even want to provide functions with this type of limitation?
Mike
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Functions that require interrupts be enabled
2007-05-15 22:33 Functions that require interrupts be enabled Mike Mason
@ 2007-05-16 0:49 ` Frank Ch. Eigler
2007-05-16 19:00 ` Mike Mason
0 siblings, 1 reply; 4+ messages in thread
From: Frank Ch. Eigler @ 2007-05-16 0:49 UTC (permalink / raw)
To: Mike Mason; +Cc: SystemTAP
Mike Mason <mmlnx@us.ibm.com> writes:
> I want to add a function to the task.stp tapset file that grabs a
> process' arguments from its user address space.
OK.
> We couldn't do this before because all probes ran with interrupts
> disabled and couldn't sleep.
Why must this new routine be permitted to sleep? We can tolerate
paged-out data via soft errors (=> blank strings).
> Now that begin/end probes no longer require that interrupts be
> disabled, this function can be used in begin/end probes at least.
AFAIK, interrupts being enabled is not exactly the same thing as being
able to sleep.
Not that this is a good idea, but:
> How do I prevent the function from being used in other probes? Is
> there a way to detect if interrupts are disabled
in_interrupt()
> or detect that the function was called from a begin/end probe?
CONTEXT->probe_point
> Do we even want to provide functions with this type of limitation?
Not really.
- FChE
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Functions that require interrupts be enabled
2007-05-16 0:49 ` Frank Ch. Eigler
@ 2007-05-16 19:00 ` Mike Mason
2007-05-17 19:46 ` Frank Ch. Eigler
0 siblings, 1 reply; 4+ messages in thread
From: Mike Mason @ 2007-05-16 19:00 UTC (permalink / raw)
To: Frank Ch. Eigler; +Cc: SystemTAP
Frank Ch. Eigler wrote:
> Mike Mason <mmlnx@us.ibm.com> writes:
>
>> I want to add a function to the task.stp tapset file that grabs a
>> process' arguments from its user address space.
>
> OK.
>
>> We couldn't do this before because all probes ran with interrupts
>> disabled and couldn't sleep.
>
> Why must this new routine be permitted to sleep? We can tolerate
> paged-out data via soft errors (=> blank strings).
My script filters based on the arguments, so having the routine randomly fail to return the actual arguments isn't good. Plus I don't think whether the page is paged in or not is an issue. I think the routine I'm using handles that (see below).
>
>> Now that begin/end probes no longer require that interrupts be
>> disabled, this function can be used in begin/end probes at least.
>
> AFAIK, interrupts being enabled is not exactly the same thing as being
> able to sleep.
Here's the routine I'm using to grab the arguments from user space. It's a modified version of access_process_vm(), which isn't callable from a module. It can potentially sleep in two places: down_read() and kmap(). These functions do a might_sleep() check and fail if interrupts are disabled. I considered using down_read_trylock() and kmap_atomic() (which won't sleep) but I don't clearly understand the side-effects of doing so. Any suggestions would be appreciated.
int _read_process_args(struct task_struct *tsk, void *buf)
{
struct mm_struct *mm;
struct vm_area_struct *vma;
struct page *page;
void *old_buf = buf;
int len;
unsigned long addr;
mm = get_task_mm(tsk);
if (!mm)
return 0;
if (!mm->arg_end)
goto out_mm;
len = mm->arg_end - mm->arg_start;
if (len > MAXSTRINGLEN)
len = MAXSTRINGLEN;
addr = mm->arg_start;
down_read(&mm->mmap_sem); /* might_sleep */
/* ignore errors, just check how much was successfully transfered */
while (len) {
int bytes, ret, offset;
void *maddr;
ret = get_user_pages(tsk, mm, addr, 1, 0, 1, &page, &vma);
if (ret <= 0)
break;
bytes = len;
offset = addr & (PAGE_SIZE - 1);
if (bytes > PAGE_SIZE - offset)
bytes = PAGE_SIZE - offset;
maddr = kmap(page); /* might_sleep */
copy_from_user_page(vma, page, addr, buf, maddr + offset, bytes);
kunmap(page);
page_cache_release(page);
len -= bytes;
buf += bytes;
addr += bytes;
}
up_read(&mm->mmap_sem);
out_mm:
mmput(mm);
return buf - old_buf;
}
>
>
> Not that this is a good idea, but:
>
>> How do I prevent the function from being used in other probes? Is
>> there a way to detect if interrupts are disabled
>
> in_interrupt()
>
>> or detect that the function was called from a begin/end probe?
>
> CONTEXT->probe_point
>
>> Do we even want to provide functions with this type of limitation?
>
> Not really.
>
> - FChE
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Functions that require interrupts be enabled
2007-05-16 19:00 ` Mike Mason
@ 2007-05-17 19:46 ` Frank Ch. Eigler
0 siblings, 0 replies; 4+ messages in thread
From: Frank Ch. Eigler @ 2007-05-17 19:46 UTC (permalink / raw)
To: Mike Mason; +Cc: systemtap
Hi -
On Wed, May 16, 2007 at 12:00:38PM -0700, Mike Mason wrote:
> [...]
> >Why must this new routine be permitted to sleep? We can tolerate
> >paged-out data via soft errors (=> blank strings).
>
> My script filters based on the arguments, so having the routine randomly
> fail to return the actual arguments isn't good.
Yeah, though I wonder how frequently that can happens, considering
that running tools like "top" would keep those pages around.
> Plus I don't think whether the page is paged in or not is an issue.
> I think the routine I'm using handles that (see below).
Yes, via possible sleeps during the other routines.
> >
> >>Now that begin/end probes no longer require that interrupts be
> >>disabled, this function can be used in begin/end probes at least.
> >
> >AFAIK, interrupts being enabled is not exactly the same thing as being
> >able to sleep.
>
> Here's the routine I'm using to grab the arguments from user space. It's a
> modified version of access_process_vm(), which isn't callable from a
> module. It can potentially sleep in two places: down_read() and kmap().
> These functions do a might_sleep() check and fail if interrupts are
> disabled.
> I considered using down_read_trylock() and kmap_atomic() (which
> won't sleep) but I don't clearly understand the side-effects of
> doing so. Any suggestions would be appreciated.
I too only have limited understanding of this. For the _trylock, that
is not a problem, as one can detect contention and return early rather
than blocking.
For kmap_atomic(), it seems trickier. You'd need to use one of the
KM_USER[01] slots. But we're in trouble if we are running this code
within a probe in a kernel routine that is already using that slot.
Then we still have this stuff that might sleep indirectly via a page
fault (unless I'm mistaken):
> ret = get_user_pages(tsk, mm, addr, 1, 0, 1, &page, &vma);
> copy_from_user_page(vma, page, addr, buf, maddr + offset, bytes);
So it seems like our options are:
(a) write such a tapset function, permit it to only be called from
begin/end-like probe contexts, and let it sleep and whatnot
(b) write a related tapset function, which uses very conservative
atomic routines everywhere, and may return blanks
(c) accept an approximation, such as a deferred result. It might
look like this:
# ------------------------------------------------------------------------
# tapset/proc-args.stp
%{
// helper C code
%}
void deferred_lookup(...) %{
// enqueue a possibly-sleepy lookup of process args using
// auxiliary thread or defer_work type callback
// arrange to put results into _process_args[] when available
// return right away
%}
global _process_args
function process_args:string (pid) {
if (pid in _process_args) return _process_args [pid]
else { deferred_lookup (pid)
return "" }
}
# ------------------------------------------------------------------------
Then, ordinary user scripts would just call process_args(NNN), and
would have to accept & ignore empty results. Eventually valid strings
should come by.
- FChE
^ permalink raw reply [flat|nested] 4+ messages in thread
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2007-05-15 22:33 Functions that require interrupts be enabled Mike Mason
2007-05-16 0:49 ` Frank Ch. Eigler
2007-05-16 19:00 ` Mike Mason
2007-05-17 19:46 ` Frank Ch. Eigler
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