Messages in this thread | | | Date | Fri, 27 Jan 2017 08:51:11 +0200 | From | Jarkko Sakkinen <> | Subject | Re: [PATCH 2/2] tpm2-space: add handling for global session exhaustion |
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On Thu, Jan 26, 2017 at 04:45:52PM -0800, James Bottomley wrote: > On Thu, 2017-01-26 at 14:56 +0200, Jarkko Sakkinen wrote: > > On Mon, Jan 23, 2017 at 09:38:33PM -0800, James Bottomley wrote: > > > In a TPM2, sessions can be globally exhausted once there are > > > TPM_PT_ACTIVE_SESSION_MAX of them (even if they're all context > > > saved). The Strategy for handling this is to keep a global count of > > > all the sessions along with their creation time. Then if we see > > > the TPM run out of sessions (via the TPM_RC_SESSION_HANDLES) we > > > first wait for one to become free, but if it doesn't, we forcibly > > > evict an existing one. The eviction strategy waits until the > > > current command is repeated to evict the session which should > > > guarantee there is an available slot. > > > > > > On the force eviction case, we make sure that the victim session is > > > at least SESSION_TIMEOUT old (currently 2 seconds). The wait queue > > > for session slots is a FIFO one, ensuring that once we run out of > > > sessions, everyone will get a session in a bounded time and once > > > they get one, they'll have SESSION_TIMEOUT to use it before it may > > > be subject to eviction. > > > > > > Signed-off-by: James Bottomley < > > > James.Bottomley@HansenPartnership.com> > > > > This is not a proper review yet. Just quick question: why do you need > > a real time (i.e. created)? Maybe in the force eviction case it would > > be enough to sleep lets say 500 ms and pick the victim with smallest > > number? I.e. just have increasing u64 counter instead of real time. > > So that if the oldest session has already been around for > 2s there's > no need to wait. In order to guarantee everyone gets a session for at > least 2s without tracking the age of sessions, you'd have to sleep for > 2s after you find the oldest session. > > > That would simplify this patch a lot and does not prevent to refine > > later on if workloads show need for more complex logic. > > An increasing monotonic counter would actually not be much simpler: all > you could really cut out would be the four lines and two comments at > the bottom of the for loop in tpm2_session_wait() which check the age > of the found session. > > James
Right. Thanks for explaining this. I'll check the code with more detail ASAP.
/Jarkko
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