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

["Ramia, Kannan Babu" <kannan.babu.ramia@intel.com>]

The problem still remains if one of the process gets abruptly killed, then the corresponding refcount will not get decremented. There should be some scavenging function to be implemented to check the process liveliness.  

regards
Kannan Babu

-----Original Message-----
From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Neil Horman
Sent: Wednesday, October 29, 2014 7:58 PM
To: Richardson, Bruce
Cc: dev@dpdk.org
Subject: Re: [dpdk-dev] [PATCH] add free hugepage function

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.
> 
>