Re: [Patch 2/2] UefiCpuPkg/MpInitLib: Remove redundant parameter.

[Laszlo Ersek <lersek@redhat.com>]

Hi Eric,

On 06/28/18 13:29, Eric Dong wrote:
> Parameter StartCount duplicates with RunningCount. After this change,
> RunningCount means the running AP count.
>
> Done Tests:
> 1.PI SCT Test
> 2.Boot OS / S3
>
> Cc: Ruiyu Ni <ruiyu.ni@intel.com>
> Cc: Jeff Fan <vanjeff_919@hotmail.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Eric Dong <eric.dong@intel.com>
> ---
>  UefiCpuPkg/Library/MpInitLib/MpLib.c | 11 +++++------
>  UefiCpuPkg/Library/MpInitLib/MpLib.h |  1 -
>  2 files changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> index 3945771764..52c9679099 100644
> --- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
> +++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> @@ -1400,7 +1400,7 @@ CheckAllAPs (
>      // value of state after setting the it to CpuStateFinished, so BSP can safely make use of its value.
>      //
>      if (GetApState(CpuData) != CpuStateBusy) {
> -      CpuMpData->RunningCount ++;
> +      CpuMpData->RunningCount --;
>        CpuMpData->CpuData[ProcessorNumber].Waiting = FALSE;
>
>        //
> @@ -1425,7 +1425,7 @@ CheckAllAPs (
>    //
>    // If all APs finish, return EFI_SUCCESS.
>    //
> -  if (CpuMpData->RunningCount == CpuMpData->StartCount) {
> +  if (CpuMpData->RunningCount == 0) {
>      return EFI_SUCCESS;
>    }
>
> @@ -1442,7 +1442,7 @@ CheckAllAPs (
>      //
>      if (CpuMpData->FailedCpuList != NULL) {
>        *CpuMpData->FailedCpuList =
> -         AllocatePool ((CpuMpData->StartCount - CpuMpData->FinishedCount + 1) * sizeof (UINTN));
> +         AllocatePool ((CpuMpData->RunningCount + 1) * sizeof (UINTN));
>        ASSERT (*CpuMpData->FailedCpuList != NULL);
>      }
>      ListIndex = 0;
> @@ -2121,7 +2121,7 @@ StartupAllAPsWorker (
>      return EFI_NOT_STARTED;
>    }
>
> -  CpuMpData->StartCount = 0;
> +  CpuMpData->RunningCount = 0;
>    for (ProcessorNumber = 0; ProcessorNumber < ProcessorCount; ProcessorNumber++) {
>      CpuData = &CpuMpData->CpuData[ProcessorNumber];
>      CpuData->Waiting = FALSE;
> @@ -2131,7 +2131,7 @@ StartupAllAPsWorker (
>          // Mark this processor as responsible for current calling.
>          //
>          CpuData->Waiting = TRUE;
> -        CpuMpData->StartCount++;
> +        CpuMpData->RunningCount++;
>        }
>      }
>    }
> @@ -2140,7 +2140,6 @@ StartupAllAPsWorker (
>    CpuMpData->ProcArguments = ProcedureArgument;
>    CpuMpData->SingleThread  = SingleThread;
>    CpuMpData->FinishedCount = 0;
> -  CpuMpData->RunningCount  = 0;
>    CpuMpData->FailedCpuList = FailedCpuList;
>    CpuMpData->ExpectedTime  = CalculateTimeout (
>                                 TimeoutInMicroseconds,
> diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.h b/UefiCpuPkg/Library/MpInitLib/MpLib.h
> index 90c09fb8fb..4166734687 100644
> --- a/UefiCpuPkg/Library/MpInitLib/MpLib.h
> +++ b/UefiCpuPkg/Library/MpInitLib/MpLib.h
> @@ -210,7 +210,6 @@ struct _CPU_MP_DATA {
>    UINTN                          BackupBuffer;
>    UINTN                          BackupBufferSize;
>
> -  volatile UINT32                StartCount;
>    volatile UINT32                FinishedCount;
>    volatile UINT32                RunningCount;
>    BOOLEAN                        SingleThread;
>

I'm currently missing a good understanding of how these counters are
modified. They are all qualified "volatile", which suggests they are
accessed from multiple processors. Is that correct?

Here's my concern: assume we have 3 APs, and we maintain how many of
them are running concurrently at any given point. Assume we start them
up all at once, and then later they all finish. The modified
"RunningCount" value might advance like this:

  AP#1  AP#2  AP#3  RunningCount  comment
  ----  ----  ----  ------------  ----------------
                    0             no AP running yet
   v                1             AP#1 starts
   |     v          2             AP#2 starts
   |     |     v    3             AP#3 starts
   |     |     |    3             all APs working
   |     ^     |    2             AP#2 finishes
   |           ^    1             AP#3 finishes
   ^                0             AP#1 finishes, done

However, the following could happen as well:

  AP#1  AP#2  AP#3  RunningCount  comment
  ----  ----  ----  ------------  ----------------
                    0             no AP running yet
   v                1             AP#1 starts
   ^                0             AP#1 finishes
         v          1             AP#2 starts
         |     v    2             AP#3 starts
         |     ^    1             AP#3 finishes
         ^          0             AP#2 finishes, done

In the second scheduling, we get RunningCount=0 when AP#1 finishes, even
though AP#2 and AP#3 are not done (they haven't even started yet).

Is this a realistic concern, or is the above scenario impossible?

(I'd like to test the series after understanding this.)

Actually... if the problem scenario is possible, I think it could affect
the current (pre-patch) code as well. I hope I'm wrong!

Thanks,
Laszlo