mem-ruby: Prevent LL/SC livelock in MESI protocols (#1384) #1399
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BobbyRBruce
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Jul 31, 2024
MESI_Two_Level and MESI_Three_Level protocols are susceptible to LL/SC livelocks when simulating boards with high core count. This fix is based on MOESI_CMP_directory's implementation of locked states, but tailors the solution to only apply it when a Load-Linked is initiated. There are two new states to act as locked states and stall any messages leading to eviction: * LLSC_E: equivalent to E state, go to E after timeout. * LLSC_M: equivalent to M state, go to M after timeout. The main new event is Load_Linked, which is very similar (in behavior) to a Store, reusing several transient states. When a controller receives the exclusive data, it differentiates a Load_Linked from a Store by checking a new field added to the TBE: 'isLoadLinked'. It triggers a different event when it is a Load_Linked, which in turn causes the transition to one of the locked states. The entire mechanism can be turned off by setting 'use_llsc_lock' to false, and the amount of time to keep locked is defined by 'llsc_lock_timeout_latency'. Change-Id: I13f415b6b7890d51d01f23001047d2363467a814
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Looks OK to me. Since it was an Arm bug, @giactra may want to take a look. |
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Hi @giactra, since this change is related to ARM, you might want to review it. |
Thanks @ivanaamit ; you might have tagged the wrong Giacomo though ;) |
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Hi @giactra, if you have time, we’d appreciate it if you could review this PR. Even a quick review would be very helpful. Thank you! |
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@giactra : This affects ARM. Could you review this? We're ready to merge but would appreciate ARM approval. |
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Fix #1384.
MESI_Two_Level and MESI_Three_Level protocols are susceptible to LL/SC livelocks when simulating boards with high core count.
This fix is based on MOESI_CMP_directory's implementation of locked states, but tailors the solution to only apply it when a Load-Linked is initiated.
There are two new states to act as locked states and stall any messages leading to eviction:
The main new event is Load_Linked, which is very similar (in behavior) to a Store, reusing several transient states. When a controller receives the exclusive data, it differentiates a Load_Linked from a Store by checking a new field added to the TBE: 'isLoadLinked'. It triggers a different event when it is a Load_Linked, which in turn causes the transition to one of the locked states.
The entire mechanism can be turned off by setting 'use_llsc_lock' to false, and the amount of time to keep locked is defined by 'llsc_lock_timeout_latency'.
Change-Id: I13f415b6b7890d51d01f23001047d2363467a814