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.bottom...@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
