netdev - Re: [PATCH 2/3] rhashtable-test: retry insert operations in threads

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Message-ID: <20150901134310.GB27550@orbit.nwl.cc>
Date:	Tue, 1 Sep 2015 15:43:11 +0200
From:	Phil Sutter <phil@....cc>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	tgraf@...g.ch, davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, fengguang.wu@...el.com,
	wfg@...ux.intel.com, lkp@...org
Subject: Re: [PATCH 2/3] rhashtable-test: retry insert operations in threads

On Tue, Sep 01, 2015 at 09:00:57PM +0800, Herbert Xu wrote:
> On Tue, Sep 01, 2015 at 02:46:48PM +0200, Phil Sutter wrote:
> >
> > This is not an inherent behaviour of the implementation but general
> > agreement. The insertion may fail non-permanently (returning -EBUSY),
> > users are expected to handle this by retrying the operation.
> 
> Absolutely not.  The only reason for an insertion to fail is if we
> can't allocate enough memory.  Unless the user is also looping its
> kmalloc calls it definitely shouldn't be retrying the insert.

rhashtable_insert_fast() returns -EBUSY if the table is full
(rht_grow_above_100() returns true) and an asynchronous rehash operation
is active. AFAICT, this is not necessarily caused by memory pressure.

> If an expansion fails it means either that the system is suffering
> a catastrophic memory shortage, or the user of rhashtable is doing
> something wrong.

Hmm. Since memory allocation is first tried with GFP_ATOMIC set and upon
failure retried in background, this seems like a situation which might
happen during normal use. If that already indicates a severe problem,
why retry in background at all?

Cheers, Phil
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