Re: [dpdk-dev] [PATCH] add free hugepage function

[Neil Horman <nhorman@tuxdriver.com>]

On Wed, Oct 29, 2014 at 10:26:35AM +0000, Bruce Richardson wrote:
> On Wed, Oct 29, 2014 at 02:49:05PM +0800, Linhaifeng wrote:
> > 
> > 
> > On 2014/10/29 13:26, Qiu, Michael wrote:
> > > 在 10/29/2014 11:46 AM, Matthew Hall 写道:
> > >> On Wed, Oct 29, 2014 at 03:27:58AM +0000, Qiu, Michael wrote:
> > >>> I just saw one return path with value '0', and no any other place 
> > >>> return a negative value,  so it is better to  be designed as one
> > >>> non-return function,
> > >>>
> > >>> +void
> > >>> +rte_eal_hugepage_free(void)
> > >>> +{
> > >>> +	struct hugepage_file *hugepg_tbl = g_hugepage_table.hugepg_tbl;
> > >>> +	unsigned i;
> > >>> +	unsigned nr_hugefiles = g_hugepage_table.nr_hugefiles;
> > >>> +
> > >>> +	RTE_LOG(INFO, EAL, "unlink %u hugepage files\n", nr_hugefiles);
> > >>> +
> > >>> +	for (i = 0; i < nr_hugefiles; i++) {
> > >>> +		unlink(hugepg_tbl[i].filepath);
> > >>> +		hugepg_tbl[i].orig_va = NULL;
> > >>> +	}
> > >>> +}
> > >>> +
> > >>>
> > >>> Thanks,
> > >>> Michael
> > >> Actually, I don't think that's quite right.
> > >>
> > >> http://linux.die.net/man/2/unlink
> > >>
> > >> "On success, zero is returned. On error, -1 is returned, and errno is set 
> > >> appropriately." So it should be returning an error, and logging a message for 
> > >> a file it cannot unlink or people will be surprised with weird failures.
> > > 
> > > Really need one message for unlink failed, but I'm afraid that if it
> > > make sense for return an error code when application exit.
> > > 
> > > Thanks
> > > Michael
> > >> It also had some minor typos / English in the comments but we can fix that too.
> > >>
> > >> Matthew.
> > >>
> > > 
> > > 
> > > 
> > Agree.May be it is not need to return error?
> > -- 
> > Regards,
> > Haifeng
> > 
> 
> May I throw out a few extra ideas.
> 
> The basic problem with DPDK and hugepages on exit, as you have already 
> diagnosed, is not that the hugepages aren't released for re-use, its that 
> the files inside the hugepage directory aren't unlinked. There are two 
I agree, this patch seems rather unbalanced.  Hugepages are initalized on
startup, not allocated explicitly, and so should not be freed on exit.  Instead,
they should be globally refcounted and (potentially per configuration) freed
when the last application to exit drops the refcount to zero.

> considerations here that prevented us from coming up previously with a 
> solution to this that we were fully happy with.
> 1. There is no automatic way (at least none that I'm aware of), to have the 
> kernel automatically delete a file on exit. This means that any scheme to 
> delete the files on application termination will have to be a voluntary one 
> that won't happen if an app crashed or is forcibly terminated. That is why 
> the EAL right now does the clean-up on the restart of the app instead.
Thats not true.  The common POSIX compliant method is to use the atexit()
function to register a function to be called when a process terminates.  It can
be used from rte_eth_hugepage_init so that a refcount is increased there and
decreased when the process exits.  The atexit registered function can also check
the refcount for zero and, dependent on configuration, unlink the hugepage
files.  That way the application can be completely unaware of a need to do
freeing/unlinking

> 2. The other consideration is multi-process. In a multi-process environment, 
> it is unclear when exactly an app is finished - just because one process 
> terminates does not mean that the whole app is finished with the hugepage 
> files. If the files are deleted before a DPDK secondary process starts up, 
> it won't be able to start as it won't be able to mmap the hugepage memory it 
> needs.
> 
Refcounting fixes that.  If every process takes a reference count on the
hugepage set, then you unlink it when the refcount hits zero.

> Now, that being said, I think we could see about having automatic hugepage 
> deletion controlled by an EAL parameter. That way, the parameter could be 
> omitted in the multi-process case, but could be used for those who want to 
> have a clean hugepage dir after app termination.
> 
> What I think we could do is, instead of having the app save the list of 
> hugepages it uses as the app initializes, is to have the app delete the 
> hugepage files as soon as it closes the filehandle for them. If my 
> understanding is correct, the kernel will actually keep the hugepage memory 
> around in the app as usual from that point onwards, and will only release it 
> back [automatically] on app termination. New processes won't be able to mmap 
> the file, but the running process will be unaffected. The best thing about 
> this approach is that the hugepage file entries will be deleted whether or 
> not the application terminates normally or crashes.
> 
That doesn't really work in a use case in which processes are created and
removed (i.e. if you have two processes, one exits, then another one is forked,
that third process won't find the hugepage file, because the second process will
have unlinked it on exit).

> So, any thoughts? Would such a scheme work? I haven't prototyped it, yet, 
> but I think it should work.
> 
> Regards,
> /Bruce
> 
> PS: As well as multi-process not being able to use this scheme, I'd also 
> note that this would prevent the dump_cfg utility app - or any other DPDK 
> analysis app like it - from being used to inspect the memory area of the 
> running process. That is another reason why I think the flag should not be 
> set by default.
> 
>