[Yaffs] [patch] latency improvements on non-preemptive linux…

(This is a resend of a message I sent last week, but I wasn't subscribed
to the list at the time so it ended up in list moderator approval land.
Apologies in advance if the original message does eventually show up as
a duplicate, but I suspect they are buried in a bucket of endless spam
and will never be heard from again...)

We noticed serious latency issues when performing NAND I/O on our
embedded target with a non-preemptive kernel -- on the order of hundreds
of milliseconds during heavy I/O. Of course enabling kernel preemption
would fix this but that had other undesirable effects.

Adding cond_resched() to strategic places in both the MTD code and yaffs
code has reduced the latency to acceptable levels for us, here is the
yaffs part of the patch if anybody is interested.

--- yaffs2-20081017/yaffs_guts.c    2008-07-20 21:03:19.000000000 -0400
+++ yaffs2-20081017-patched/yaffs_guts.c    2008-10-28 
14:50:00.000000000 -0400
@@ -714,6 +714,7 @@
                }
            }
        }
+        YYIELD();

    }

@@ -3141,6 +3142,7 @@
                yaffs_DeleteChunk(dev, oldChunk, markNAND, __LINE__);

            }
+            YYIELD();
        }

        yaffs_ReleaseTempBuffer(dev, buffer, __LINE__);
@@ -3247,6 +3249,7 @@
               ("yaffs: GC !!!no reclaim!!! erasedBlocks %d after try %d 
block %d"
                TENDSTR), dev->nErasedBlocks, maxTries, block));
        }
+        YYIELD();
    } while ((dev->nErasedBlocks < dev->nReservedBlocks) && (block > 0)
         && (maxTries < 2));

@@ -3889,6 +3892,7 @@
                cache->dirty = 0;
                cache->object = NULL;
            }
+            YYIELD();

        } while (cache && chunkWritten > 0);

@@ -3926,6 +3930,7 @@
        }
        if(obj)
            yaffs_FlushFilesChunkCache(obj);
+        YYIELD();

    } while(obj);

@@ -4780,6 +4785,7 @@
        buffer += nToCopy;
        nDone += nToCopy;

+        YYIELD();
    }

    return nDone;
@@ -4965,6 +4971,7 @@
            buffer += nToCopy;
            nDone += nToCopy;
        }
+        YYIELD();

    }